β-Catenin Activation in Muscle Progenitor Cells Regulates Tissue Repair

Crosstalk among canonical Wnt and Hippo pathway members in skeletal muscle and at the neuromuscular junction

Skeletal muscles are essential for locomotion, posture, and metabolic regulation. To understand physiological processes, exercise adaptation, and muscle-related disorders, it is critical to understand the molecular pathways that underlie skeletal muscle function. The process of muscle contraction, orchestrated by a complex interplay of molecular events, is at the core of skeletal muscle function. Muscle contraction is initiated by an action potential and neuromuscular transmission requiring a neuromuscular junction. Within muscle fibers, calcium ions play a critical role in mediating the interaction between actin and myosin filaments that generate force. Regulation of calcium release from the sarcoplasmic reticulum plays a key role in excitation-contraction coupling. The development and growth of skeletal muscle are regulated by a network of molecular pathways collectively known as myogenesis. Myogenic regulators coordinate the differentiation of myoblasts into mature muscle fibers. Signaling pathways regulate muscle protein synthesis and hypertrophy in response to mechanical stimuli and nutrient availability. Several muscle-related diseases, including congenital myasthenic disorders, sarcopenia, muscular dystrophies, and metabolic myopathies, are underpinned by dysregulated molecular pathways in skeletal muscle. Therapeutic interventions aimed at preserving muscle mass and function, enhancing regeneration, and improving metabolic health hold promise by targeting specific molecular pathways. Other molecular signaling pathways in skeletal muscle include the canonical Wnt signaling pathway, a critical regulator of myogenesis, muscle regeneration, and metabolic function, and the Hippo signaling pathway. In recent years, more details have been uncovered about the role of these two pathways during myogenesis and in developing and adult skeletal muscle fibers, and at the neuromuscular junction. In fact, research in the last few years now suggests that these two signaling pathways are interconnected and that they jointly control physiological and pathophysiological processes in muscle fibers. In this review, we will summarize and discuss the data on these two pathways, focusing on their concerted action next to their contribution to skeletal muscle biology. However, an in-depth discussion of the non-canonical Wnt pathway, the fibro/adipogenic precursors, or the mechanosensory aspects of these pathways is not the focus of this review.

Deciphering the role of IGFBP5 in delaying fibrosis and sarcopenia in aging skeletal muscle: therapeutic implications and molecular mechanisms

Introduction

Sarcopenia is a condition characterized by the loss of muscle fibers and excessive deposition of extracellular matrix proteins. The interplay between muscle atrophy and fibrosis is a central feature of sarcopenia. While the mechanisms underlying skeletal muscle aging and fibrosis remain incompletely understood, cellular senescence has emerged as a key contributor. This study investigates the role of D-galactose (D-gal) in inducing fibroblasts senescence and skeletal muscle fibrosis, and aims to find the key regulator of the process to serve as a therapeutical target.

Methods

To discover the role of D-gal in inducing cellular senescence and fibrosis, the senescence markers and the expression of fibrosis-related proteins were assessed after introducing D-gal among fibroblasts, and muscle strength and mass. The severity of muscle atrophy and fibrosis were also verified by using H&E staining and Masson trichrome staining after D-gal treatment via subcutaneous injection among mice. Subsequently, mRNA sequencing (RNA-seq) was performed and the differential expressed genes were identified between under D-gal or control treatment, to discover the key regulator of D-GAL-driven fibroblasts senescence and fibrosis. The role of the key regulator IGFBP5 were then validated in D-GAL treated IGFBP5-knockdown fibroblasts in vitro by analyzing the level of senescence and fibrosis-related markers. And the results were further confirmed in vivo in IGFBP5-knockdown SAMP8 mice with histological examinations.

Results

D-gal treatment effectively induced cellular senescence and fibrosis in fibroblasts, as well as skeletal muscle atrophy, fibrosis and loss in muscle mass and function in mice. IGFBP5 was identified as a key regulator of D-GAL induced senescence and fibrosis among fibroblasts using RNA-seq. And further validation tests showed that IGFBP5-knockdown could alleviate D-GAL-induced fibroblast cellular senescence and fibrosis, as well as the severity of muscle atrophy and fibrosis in SAMP8 mice.

Discussion

IGFBP5 emerging as a key regulator of D-GAL-induced fibroblast cellular senescence and fibrosis. The findings provide new insights into the molecular mechanisms underlying age-related skeletal muscle fibrosis and highlight IGFBP5 as a potential therapeutic target. Further research is needed to validate these findings and explore related clinical applications.

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

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

Optimisation of cell fate determination for cultivated muscle differentiation

Production of cultivated meat requires defined medium formulations for the robust differentiation of myogenic cells into mature skeletal muscle fibres in vitro. Although these formulations can drive myogenic differentiation levels comparable to serum-starvation-based protocols, the resulting cultures are often heterogeneous, with a significant proportion of cells not participating in myofusion, limiting maturation of the muscle. To address this problem, we employed RNA sequencing to analyse heterogeneity in differentiating bovine satellite cells at single-nucleus resolution, identifying distinct cellular subpopulations including proliferative cells that fail to exit the cell cycle and quiescent 'reserve cells' that do not commit to myogenic differentiation. Our findings indicate that the MEK/ERK, NOTCH, and RXR pathways are active during the initial stages of myogenic cell fate determination, and by targeting these pathways, we can promote cell cycle exit while reducing reserve cell formation. This optimised medium formulation consistently yields fusion indices close to 100% in 2D culture. Furthermore, we demonstrate that these conditions enhance myotube formation and actomyosin accumulation in 3D bovine skeletal muscle constructs, providing proof of principle for the generation of highly differentiated cultivated muscle with excellent mimicry to traditional muscle.

Delta-like noncanonical notch ligand 2 regulates the proliferation and differentiation of sheep myoblasts through the Wnt/β-catenin signaling pathway

This study delved into the role of delta-like noncanonical notch ligand 2 (DLK2) in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts, as well as its interaction with the classical Wnt/β-catenin signaling pathway in regulating myoblast function. The research revealed that upregulation of DLK2 in myoblasts during the proliferation phase enhanced myoblast proliferation, facilitated cell cycle progression, and reduced apoptosis. Conversely, downregulation of DLK2 expression using siRNA during the differentiation phase promoted myoblast hypertrophy and fusion, suppressed the expression of muscle fiber degradation factors, and expedited the differentiation process. DLK2 regulates myoblasts function by influencing the expression of various factors associated with the Wnt/β-catenin signaling pathway, including CTNNB1, FZD1, FZD6, RSPO1, RSPO4, WNT4, WNT5A, and adenomatous polyposis coli. In essence, DLK2, with the involvement of the Wnt/β-catenin signaling pathway, plays a crucial regulatory role in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts.

CLIC5 promotes myoblast differentiation and skeletal muscle regeneration via the BGN-mediated canonical Wnt/β-catenin signaling pathway

Myoblast differentiation plays a vital role in skeletal muscle regeneration. However, the protein-coding genes controlling this process remain incompletely understood. Here, we showed that chloride intracellular channel 5 (CLIC5) exerts a critical role in mediating myogenesis and skeletal muscle regeneration. Deletion of CLIC5 in skeletal muscle leads to reduced muscle weight and decreases the number and differentiation potential of satellite cells. In vitro, CLIC5 consistently inhibits myoblast proliferation while promoting myotube formation. CLIC5 promotes myogenic differentiation by activating the canonical Wnt/β-catenin signaling pathway in a biglycan (BGN)-dependent manner. CLIC5 deletion impairs muscle regeneration. Paired box gene 7 (Pax7) expression and the activity of BGN-mediated canonical Wnt/β-catenin signaling are reduced in CLIC5-deficient mice. Conversely, increasing CLIC5 levels in skeletal muscles enhances muscle regeneration capacity. In conclusion, our findings underscore CLIC5 as a pivotal regulator of myogenesis and skeletal muscle regeneration, functioning through interaction with BGN to activate the canonical Wnt/β-catenin signaling pathway.

We need to talk-how muscle stem cells communicate

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

Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

SETDB1 modulates the TGFβ response in Duchenne muscular dystrophy myotubes

Overactivation of the transforming growth factor-β (TGFβ) signaling in Duchenne muscular dystrophy (DMD) is a major hallmark of disease progression, leading to fibrosis and muscle dysfunction. Here, we investigated the role of SETDB1 (SET domain, bifurcated 1), a histone lysine methyltransferase involved in muscle differentiation. Our data show that, following TGFβ induction, SETDB1 accumulates in the nuclei of healthy myotubes while being already present in the nuclei of DMD myotubes where TGFβ signaling is constitutively activated. Transcriptomics revealed that depletion of SETDB1 in DMD myotubes leads to down-regulation of TGFβ target genes coding for secreted factors involved in extracellular matrix remodeling and inflammation. Consequently, SETDB1 silencing in DMD myotubes abrogates the deleterious effect of their secretome on myoblast differentiation by impairing myoblast pro-fibrotic response. Our findings indicate that SETDB1 potentiates the TGFβ-driven fibrotic response in DMD muscles, providing an additional axis for therapeutic intervention.

Targeting Wnt/β-catenin signaling and its interplay with TGF-β and Notch signaling pathways for the treatment of chronic wounds

Wound healing is a tightly regulated process that ensures tissue repair and normal function following injury. It is modulated by activation of pathways such as the transforming growth factor-beta (TGF-β), Notch, and Wnt/β-catenin signaling pathways. Dysregulation of this process causes poor wound healing, which leads to tissue fibrosis and ulcerative wounds. The Wnt/β-catenin pathway is involved in all phases of wound healing, primarily in the proliferative phase for formation of granulation tissue. This review focuses on the role of the Wnt/β-catenin signaling pathway in wound healing, and its transcriptional regulation of target genes. The crosstalk between Wnt/β-catenin, Notch, and the TGF-β signaling pathways, as well as the deregulation of Wnt/β-catenin signaling in chronic wounds are also considered, with a special focus on diabetic ulcers. Lastly, we discuss current and prospective therapies for chronic wounds, with a primary focus on strategies that target the Wnt/β-catenin signaling pathway such as photobiomodulation for healing diabetic ulcers.

LSD1 controls a nuclear checkpoint in Wnt/β-Catenin signaling to regulate muscle stem cell self-renewal

The Wnt/β-Catenin pathway plays a key role in cell fate determination during development and in adult tissue regeneration by stem cells. These processes involve profound gene expression and epigenome remodeling and linking Wnt/β-Catenin signaling to chromatin modifications has been a challenge over the past decades. Functional studies of the lysine demethylase LSD1/KDM1A converge to indicate that this epigenetic regulator is a key regulator of cell fate, although the extracellular cues controlling LSD1 action remain largely unknown. Here we show that β-Catenin is a substrate of LSD1. Demethylation by LSD1 prevents β-Catenin degradation thereby maintaining its nuclear levels. Consistently, in absence of LSD1, β-Catenin transcriptional activity is reduced in both MuSCs and ESCs. Moreover, inactivation of LSD1 in mouse muscle stem cells and embryonic stem cells shows that LSD1 promotes mitotic spindle orientation via β-Catenin protein stabilization. Altogether, by inscribing LSD1 and β-Catenin in the same molecular cascade linking extracellular factors to gene expression, our results provide a mechanistic explanation to the similarity of action of canonical Wnt/β-Catenin signaling and LSD1 on stem cell fate.

Lysine Distinctively Manipulates Myogenic Regulatory Factors and Wnt/Ca2+ Pathway in Slow and Fast Muscles, and Their Satellite Cells of Postnatal Piglets

Muscle regeneration, representing an essential homeostatic process, relies mainly on the myogenic progress of resident satellite cells, and it is modulated by multiple physical and nutritional factors. Here, we investigated how myogenic differentiation-related factors and pathways respond to the first limiting amino acid lysine (Lys) in the fast and slow muscles, and their satellite cells (SCs), of swine. Thirty 28-day-old weaned piglets with similar body weights were subjected to three diet regimens: control group (d 0-28: 1.31% Lys, n = 12), Lys-deficient group (d 0-28: 0.83% Lys, n = 12), and Lys rescue group (d 0-14: 0.83% Lys; d 15-28: 1.31% Lys, n = 6). Pigs on d 15 and 29 were selectively slaughtered for muscular parameters evaluation. Satellite cells isolated from fast (semimembranosus) and slow (semitendinosus) muscles were also selected to investigate differentiation ability variations. We found Lys deficiency significantly hindered muscle development in both fast and slow muscles via the distinct manipulation of myogenic regulatory factors and the Wnt/Ca2+ pathway. In the SC model, Lys deficiency suppressed the Wnt/Ca2+ pathways and myosin heavy chain, myogenin, and myogenic regulatory factor 4 in slow muscle SCs but stimulated them in fast muscle SCs. When sufficient Lys was attained, the fast muscle-derived SCs Wnt/Ca2+ pathway (protein kinase C, calcineurin, calcium/calmodulin-dependent protein kinase II, and nuclear factor of activated T cells 1) was repressed, while the Wnt/Ca2+ pathway of its counterpart was stimulated to further the myogenic differentiation. Lys potentially manipulates the differentiation of porcine slow and fast muscle myofibers via the Wnt/Ca2+ pathway in opposite trends.

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

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

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

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

Absence of the primary cilia formation gene Talpid3 impairs muscle stem cell function

Skeletal muscle stem cells (MuSC) are crucial for tissue homoeostasis and repair after injury. Following activation, they proliferate to generate differentiating myoblasts. A proportion of cells self-renew, re-enter the MuSC niche under the basal lamina outside the myofiber and become quiescent. Quiescent MuSC have a primary cilium, which is disassembled upon cell cycle entry. Ex vivo experiments suggest cilia are important for MuSC self-renewal, however, their requirement for muscle regeneration in vivo remains poorly understood. Talpid3 (TA3) is essential for primary cilia formation and Hedgehog (Hh) signalling. Here we use tamoxifen-inducible conditional deletion of TA3 in MuSC (iSC-KO) and show that regeneration is impaired in response to cytotoxic injury. Depletion of MuSC after regeneration suggests impaired self-renewal, also consistent with an exacerbated phenotype in TA3iSC-KO mice after repeat injury. Single cell transcriptomics of MuSC progeny isolated from myofibers identifies components of several signalling pathways, which are deregulated in absence of TA3, including Hh and Wnt. Pharmacological activation of Wnt restores muscle regeneration, while purmorphamine, an activator of the Smoothened (Smo) co-receptor in the Hh pathway, has no effect. Together, our data show that TA3 and primary cilia are important for MuSC self-renewal and pharmacological treatment can efficiently restore muscle regeneration.

Sox11 is enriched in myogenic progenitors but dispensable for development and regeneration of the skeletal muscle

Transcription factors (TFs) play key roles in regulating differentiation and function of stem cells, including muscle satellite cells (MuSCs), a resident stem cell population responsible for postnatal regeneration of the skeletal muscle. Sox11 belongs to the Sry-related HMG-box (SOX) family of TFs that play diverse roles in stem cell behavior and tissue specification. Analysis of single-cell RNA-sequencing (scRNA-seq) datasets identify a specific enrichment of Sox11 mRNA in differentiating but not quiescent MuSCs. Consistent with the scRNA-seq data, Sox11 levels increase during differentiation of murine primary myoblasts in vitro. scRNA-seq data comparing muscle regeneration in young and old mice further demonstrate that Sox11 expression is reduced in aged MuSCs. Age-related decline of Sox11 expression is associated with reduced chromatin contacts within the topologically associating domains. Unexpectedly, Myod1Cre-driven deletion of Sox11 in embryonic myoblasts has no effects on muscle development and growth, resulting in apparently healthy muscles that regenerate normally. Pax7CreER- or Rosa26CreER- driven (MuSC-specific or global) deletion of Sox11 in adult mice similarly has no effects on MuSC differentiation or muscle regeneration. These results identify Sox11 as a novel myogenic differentiation marker with reduced expression in quiescent and aged MuSCs, but the specific function of Sox11 in myogenesis remains to be elucidated.

Sox11 is enriched in myogenic progenitors but dispensable for development and regeneration of skeletal muscle

Transcription factors (TFs) play key roles in regulating the differentiation and function of stem cells, including muscle satellite cells (MuSCs), a resident stem cell population responsible for postnatal regeneration of the skeletal muscle. Sox11 belongs to the Sry-related HMG-box (SOX) family of TFs that play diverse roles in stem cell behavior and tissue specification. Analysis of single-cell RNA-sequencing (scRNA-seq) datasets identify a specific enrichment of Sox11 mRNA in differentiating but not quiescent MuSCs. Consistent with the scRNA-seq data, Sox11 levels increase during differentiation of murine primary myoblasts in vitro. scRNA-seq data comparing muscle regeneration in young and old mice further demonstrate that Sox11 expression is reduced in aged MuSCs. Age-related decline of Sox11 expression is associated with reduced chromatin contacts within the topologically associated domains. Unexpectedly, Myod1 Cre -driven deletion of Sox11 in embryonic myoblasts has no effects on muscle development and growth, resulting in apparently healthy muscles that regenerate normally. Pax7 CreER or Rosa26 CreER driven (MuSC-specific or global) deletion of Sox11 in adult mice similarly has no effects on MuSC differentiation or muscle regeneration. These results identify Sox11 as a novel myogenic differentiation marker with reduced expression in quiescent and aged MuSCs, but the specific function of Sox11 in myogenesis remain to be elucidated.

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.

Transcriptome-Wide Study Revealed That N6-Methyladenosine Participates in Regulation Meat Production in Goats

In mammals, skeletal muscle development is a complex biological process regulated by many factors. N6-methyladenosine (m6A) RNA modification plays an important role in many biological processes. However, the regulation of m6A on skeletal muscle growth and development in adult goats remains unclear. In this study, Duan goats (DA) and Nubia goats (NBY), both female and 12 months old, were selected as the research objects, and m6A-Seq and RNA-Seq were mainly used to detect the difference of m6A modification and gene expression during the development of the longissimus dorsi (LD) muscle in the two breeds. The results showed that compared with DA, the meat production performance of NBY was better than that of DA, and the modification level of m6A was higher than that of DA in LD. The m6A-Seq of LD indicated m6A peaks were mainly enriched in the coding sequence (CDS) and stop codon. A total of 161 differentially methylated genes (DMGs) and 1294 differentially expressed genes (DEGs) were identified in two breeds. GO and KEGG analysis showed that DMGs were closely related to cellular metabolism, and most of DMGs were enriched in pathways related to energy metabolism, muscle growth and development, mainly MAPK signaling pathway, Wnt signaling pathway and CGMP-PKG signaling pathway. The DEGs were significantly enriched in actin binding, calcium ion binding, angiogenesis, and other biological processes, and most of them were enriched in PI3K-Akt and CGMP-PKG signaling pathways. Combined analysis of m6A-Seq and RNA-Seq data revealed a negative correlation between differentially methylated m6A levels and mRNA abundance, and mRNA expression of the gene with m6A peak near 3′UTR will decrease. In addition, 11 DMGs regulating cell differentiation, muscle growth and development were identified. This study displayed the m6A profiles and distribution patterns in the goat transcriptome, determined the potential role of m6A modification in muscle growth and provided a new reference for the further study of goat skeletal muscle development.

Sarcopenia: Molecular regulatory network for loss of muscle mass and function

Skeletal muscle is the foundation of human function and plays a key role in producing exercise, bone protection, and energy metabolism. Sarcopenia is a systemic disease, which is characterized by degenerative changes in skeletal muscle mass, strength, and function. Therefore, sarcopenia often causes weakness, prolonged hospitalization, falls and other adverse consequences that reduce the quality of life, and even lead to death. In recent years, sarcopenia has become the focus of in-depth research. Researchers have suggested some molecular mechanisms for sarcopenia according to different muscle physiology. These mechanisms cover neuromuscular junction lesion, imbalance of protein synthesis and breakdown, satellite cells dysfunction, etc. We summarize the latest research progress on the molecular mechanism of sarcopenia in this review in order to provide new ideas for future researchers to find valuable therapeutic targets and develop relevant prevention strategies.

Signaling pathways of adipose stem cell-derived exosomes promoting muscle regeneration

ABSTRACT

Severe muscle injury is still a challenging clinical problem. Exosomes derived from adipose stem cells (ASC-exos) may be a potential therapeutic tool, but their mechanism is not completely clear. This review aims to elaborate the possible mechanism of ASC-exos in muscle regeneration from the perspective of signal pathways and provide guidance for further study. Literature cited in this review was acquired through PubMed using keywords or medical subject headings, including adipose stem cells, exosomes, muscle regeneration, myogenic differentiation, myogenesis, wingless/integrated (Wnt), mitogen-activated protein kinases, phosphatidylinositol-4,5-bisphosphate 3-kinase/protein kinase B (PI3K/Akt), Janus kinase/signal transducers and activators of transcription, and their combinations. We obtained the related signal pathways from proteomics analysis of ASC-exos in the literature, and identified that ASC-exos make different contributions to multiple stages of skeletal muscle regeneration by those signal pathways.

Antagonism Between DUX4 and DUX4c Highlights a Pathomechanism Operating Through β-Catenin in Facioscapulohumeral Muscular Dystrophy

Aberrant expression of the transcription factor DUX4 from D4Z4 macrosatellite repeats on chromosome 4q35, and its transcriptome, associate with pathogenesis in facioscapulohumeral muscular dystrophy (FSHD). Forced DUX4 expression halts skeletal muscle cell proliferation and induces cell death. DUX4 binds DNA via two homeodomains that are identical in sequence to those of DUX4c (DUX4L9): a closely related transcriptional regulator encoded by a single, inverted, mutated D4Z4 unit located centromeric to the D4Z4 macrosatellite array on chromosome 4. However, the function and contribution of DUX4c to FSHD pathogenesis are unclear. To explore interplay between DUX4, DUX4c, and the DUX4-induced phenotype, we investigated whether DUX4c interferes with DUX4 function in human myogenesis. Constitutive expression of DUX4c rescued the DUX4-induced inhibition of proliferation and reduced cell death in human myoblasts. Functionally, DUX4 promotes nuclear translocation of β-CATENIN and increases canonical WNT signalling. Concomitant constitutive expression of DUX4c prevents β-CATENIN nuclear accumulation and the downstream transcriptional program. DUX4 reduces endogenous DUX4c levels, whereas constitutive expression of DUX4c robustly suppresses expression of DUX4 target genes, suggesting molecular antagonism. In line, DUX4 expression in FSHD myoblasts correlates with reduced DUX4c levels. Addressing the mechanism, we identified a subset of genes involved in the WNT/β-CATENIN pathway that are differentially regulated between DUX4 and DUX4c, whose expression pattern can separate muscle biopsies from severely affected FSHD patients from healthy. Finally, blockade of WNT/β-CATENIN signalling rescues viability of FSHD myoblasts. Together, our study highlights an antagonistic interplay whereby DUX4 alters cell viability via β-CATENIN signalling and DUX4c counteracts aspects of DUX4-mediated toxicity in human muscle cells, potentially acting as a gene modifier for FSHD severity. Importantly, direct DUX4 regulation of the WNT/β-CATENIN pathway informs future therapeutic interventions to ameliorate FSHD pathology.

Primary myoblasts from intrauterine growth-restricted fetal sheep exhibit intrinsic dysfunction of proliferation and differentiation that coincides with enrichment of inflammatory cytokine signaling pathways

Intrauterine growth restriction (IUGR) is linked to lifelong reductions in muscle mass due to intrinsic functional deficits in myoblasts, but the mechanisms underlying these deficits are not known. Our objective was to determine if the deficits were associated with changes in inflammatory and adrenergic regulation of IUGR myoblasts, as was previously observed in IUGR muscle. Primary myoblasts were isolated from IUGR fetal sheep produced by hyperthermia-induced placental insufficiency (PI-IUGR; n = 9) and their controls (n = 9) and from IUGR fetal sheep produced by maternofetal inflammation (MI-IUGR; n = 6) and their controls (n = 7). Proliferation rates were less (P < 0.05) for PI-IUGR myoblasts than their controls and were not affected by incubation with IL-6, TNF-α, norepinephrine, or insulin. IκB kinase inhibition reduced (P < 0.05) proliferation of control myoblasts modestly in basal media but substantially in TNF-α-added media and reduced (P < 0.05) PI-IUGR myoblast proliferation substantially in basal and TNF-α-added media. Proliferation was greater (P < 0.05) for MI-IUGR myoblasts than their controls and was not affected by incubation with TNF-α. Insulin increased (P < 0.05) proliferation in both MI-IUGR and control myoblasts. After 72-h differentiation, fewer (P < 0.05) PI-IUGR myoblasts were myogenin+ than controls in basal and IL-6 added media but not TNF-α-added media. Fewer (P < 0.05) PI-IUGR myoblasts were desmin+ than controls in basal media only. Incubation with norepinephrine did not affect myogenin+ or desmin+ percentages, but insulin increased (P < 0.05) both markers in control and PI-IUGR myoblasts. After 96-h differentiation, fewer (P < 0.05) MI-IUGR myoblasts were myogenin+ and desmin+ than controls regardless of media, although TNF-α reduced (P < 0.05) desmin+ myoblasts for both groups. Differentiated PI-IUGR myoblasts had greater (P < 0.05) TNFR1, ULK2, and TNF-α-stimulated TLR4 gene expression, and PI-IUGR semitendinosus muscle had greater (P < 0.05) TNFR1 and IL6 gene expression, greater (P < 0.05) c-Fos protein, and less (P < 0.05) IκBα protein. Differentiated MI-IUGR myoblasts had greater (P < 0.05) TNFR1 and IL6R gene expression, tended to have greater (P = 0.07) ULK2 gene expression, and had greater (P < 0.05) β-catenin protein and TNF-α-stimulated phosphorylation of NFκB. We conclude that these enriched components of TNF-α/TNFR1/NFκB and other inflammatory pathways in IUGR myoblasts contribute to their dysfunction and help explain impaired muscle growth in the IUGR fetus.

R-spondin3 is a myokine that differentiates myoblasts to type I fibres

Muscle fibres are broadly categorised into types I and II; the fibre-type ratio determines the contractile and metabolic properties of skeletal muscle tissue. The maintenance of type I fibres is essential for the prevention of obesity and the treatment of muscle atrophy caused by type 2 diabetes or unloading. Some reports suggest that myokines are related to muscle fibre type determination. We thus explored whether a myokine determines whether satellite cells differentiate to type I fibres. By examining the fibre types separately, we identified R-spondin 3 (Rspo3) as a myokine of interest, a secreted protein known as an activator of Wnt signalling pathways. To examine whether Rspo3 induces type I fibres, primary myoblasts prepared from mouse soleus muscles were exposed to a differentiation medium containing the mouse recombinant Rspo3 protein. Expression of myosin heavy chain (MyHC) I, a marker of type I fibre, significantly increased in the differentiated myotubes compared with a control. The Wnt/β-catenin pathway was shown to be the dominant signalling pathway which induces Rspo3-induced MyHC I expression. These results revealed Rspo3 as a myokine that determines whether satellite cells differentiate to type I fibres.

Bidirectional roles of skeletal muscle fibro-adipogenic progenitors in homeostasis and disease

Sarcopenia and myopathies cause progressive muscle weakness and degeneration, which are closely associated with fat infiltration and fibrosis in muscle. Recently, experimental research has shed light on fibro-adipogenic progenitors (FAPs), also known as muscle-resident mesenchymal progenitors with multiple differentiation potential for adipogenesis, fibrosis, osteogenesis and chondrogenesis. They are considered key regulators of muscle homeostasis and integrity. They play supportive roles in muscle development and repair by orchestrating the regulatory interplay between muscle stem cells (MuSCs) and immune cells. Interestingly, FAPs also contribute to intramuscular fat infiltration, fibrosis and other pathologies when the functional integrity of the network is compromised. In this review, we summarize recent insights into the roles of FAPs in maintenance of skeletal muscle homeostasis, and discuss the underlying mechanisms regulating FAPs behavior and fate, highlighting their roles in participating in efficient muscle repair and fat infiltrated muscle degeneration as well as during muscle atrophy. We suggest that controlling and predicting FAPs differentiation may become a promising strategy to improve muscle function and prevent irreparable muscle damage.

The Importance of on Sclerostin as Bone-Muscle Mediator Crosstalk

Loss of bone and muscle mass is a frequent aging condition and has become a growing public health problem. The term “osteosarcopenia” denotes close links between bone and muscle. Mechanical exercise was once thought to be the only mechanism of crosstalk between muscle and bone. Sclerostin is an important player in the process of unloading-induced bone loss and plays an important role in mechanotransduction in the bone. Furthermore, bones and muscles are categorized as endocrine organs because they produce hormone-like substances, resulting in “bone-muscle crosstalk.” Sclerostin, an inhibitor of bone development, has recently been shown to play a role in myogenesis. This review discusses the importance of sclerostin in bone-muscle crosstalk.

Myoblast deactivation within engineered human skeletal muscle creates a transcriptionally heterogeneous population of quiescent satellite-like cells

Satellite cells (SCs), the adult Pax7-expressing stem cells of skeletal muscle, are essential for muscle repair. However, in vitro investigations of SC function are challenging due to isolation-induced SC activation, loss of native quiescent state, and differentiation to myoblasts. In the present study, we optimized methods to deactivate in vitro expanded human myoblasts within a 3D culture environment of engineered human skeletal muscle tissues ("myobundles"). Immunostaining and gene expression analyses revealed that a fraction of myoblasts within myobundles adopted a quiescent phenotype (3D-SCs) characterized by increased Pax7 expression, cell cycle exit, and activation of Notch signaling. Similar to native SCs, 3D-SC quiescence is regulated by Notch and Wnt signaling while loss of quiescence and reactivation of 3D-SCs can be induced by growth factors including bFGF. Myobundle injury with a bee toxin, melittin, induces robust myofiber fragmentation, functional decline, and 3D-SC proliferation. By applying single cell RNA-sequencing (scRNA-seq), we discover the existence of two 3D-SC subpopulations (quiescent and activated), identify deactivation-associated gene signature using trajectory inference between 2D myoblasts and 3D-SCs, and characterize the transcriptomic changes within reactivated 3D-SCs in response to melittin-induced injury. These results demonstrate the ability of an in vitro engineered 3D human skeletal muscle environment to support the formation of a quiescent and heterogeneous SC population recapitulating several aspects of the native SC phenotype, and provide a platform for future studies of human muscle regeneration and disease-associated SC dysfunction.

Role of skeletal muscle satellite cells in the repair of osteoporotic fractures mediated by β-catenin

BACKGROUND

Osteoporosis is a metabolic disease, and osteoporotic fracture (OPF) is one of its most serious complications. It is often ignored that the influence of the muscles surrounding the fracture on the healing of OPF. We aimed to clarify the role of skeletal muscle satellite cells (SMSCs) in promoting OPF healing by β-catenin, to improve our understanding of SMSCs, and let us explore its potential as a therapeutic target.

METHODS

Skeletal muscles were obtained from control non-OPF or OPF patients for primary SMSCs culture (n = 3, 33% females, mean age 60 ± 15.52). Expression of SMSCs was measured. In vivo, 3-month-old female C57BL/6 mice underwent OVX surgery. Three months later, the left tibia fracture model was again performed. The control and the treatment group (n = 24, per group, female). The treatment group was treated with an agonist (osthole). Detection of SMSCs in muscles and fracture healing at 7, 14, and 28 three time points (n = 8, 8, 8, female). To further clarify the scientific hypothesis, we innovatively used Pax7-Cre^ERT2/+ ;β-catenin^fx/fx transgenic mice (n = 12, per group, male). Knock out β-catenin in SMSC to observe the proliferation and osteogenic differentiation of SMSCs, and OPF healing. In vitro primary cells of SMSCs from 3-month-old litter-negative β-catenin^fx/fx transgenic mice. After adenovirus-CRE transfection, the myogenic and osteogenic differentiation of SMSC was observed.

RESULTS

We find that human SMSCs reduced proliferation and osteogenic differentiation in patients with OPF (-38.63%, P < 0.05). And through animal experiments, it was found that activation of β-catenin promoted the proliferation and osteogenic differentiation of SMSC at the fracture site, thereby accelerating the healing of the fracture site (189.47%, P < 0.05). To prove this point of view, in the in vivo Pax7-Cre^ERT2/+ ;β-catenin^fx/fx transgenic mouse experiment, we innovatively found that knocking out β-catenin in SMSC will cause a decrease in bone mass and bone microstructure, and accompanied by delayed fracture healing (-35.04%, P < 0.001). At the same time, through in vitro SMSC culture experiments, it was found that their myogenic (-66.89%, P < 0.01) and osteogenic differentiation (-16.5%, P < 0.05) ability decreased.

CONCLUSIONS

These results provide the first practical evidence for a direct contribution of SMSCs to promote the healing of OPF with important clinical implications as it may help in the treatment of delayed healing and non-union of OPFs, and mobilization of autologous stem cell therapy in orthopaedic applications.

Muscle Stem Cell Function Is Impaired in β2-Adrenoceptor Knockout Mice

Knockout (ko) mice for the β2 adrenoceptor (Adrβ2) have impaired skeletal muscle regeneration, suggesting that this receptor is important for muscle stem cell (satellite cell) function. Here, we investigated the role of Adrβ2 in the function of satellite cells from β2ko mice in the context of muscle regeneration, through in vivo and in vitro experiments. Immunohistochemical analysis showed a significant reduction in the number of self-renewed Pax7⁺ satellite cells, proliferating Pax7⁺/MyoD⁺ myogenic precursor cells, and regenerating eMHC⁺ myofibers in regenerating muscle of β2ko mice at 30, 3, and 10 days post-injury, respectively. Quiescent satellite cells were isolated by fluorescence-activated cell sorting, and cell cycle entry was assessed by EdU incorporation. The results demonstrated a lower number of proliferating Pax7⁺/EdU⁺ satellite cells from β2ko mice. There was an increase in the gene expression of the cell cycle inhibitor Cdkn1a and Notch pathway components and the activation of Notch signaling in proliferating myoblasts from β2ko mice. There was a decrease in the number of myogenin-positive nuclei in myofibers maintained in differentiation media, and a lower fusion index in differentiating myoblasts from β2ko mice. Furthermore, the gene expression of Wnt/β-catenin signaling components, the expression of nuclear β-catenin and the activation of Wnt/β-catenin signaling decreased in differentiating myoblasts from β2ko mice. These results indicate that Adrβ2 plays a crucial role in satellite cell self-renewal, as well as in myoblast proliferation and differentiation by regulating Notch and Wnt/β-catenin signaling, respectively.

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

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

p21-activated kinase 4 phosphorylates peroxisome proliferator-activated receptor Υ and suppresses skeletal muscle regeneration

BACKGROUND

Skeletal muscle regeneration is an adaptive response to injury that is crucial to the maintenance of muscle mass and function. A p21-activated kinase 4 (PAK4) serine/threonine kinase is critical to the regulation of cytoskeletal changes, cell proliferation, and growth. However, PAK4's role in myoblast differentiation and regenerative myogenesis remains to be determined.

METHODS

We used a mouse model of myotoxin (notexin)-induced muscle regeneration. In vitro myogenesis was performed in the C2C12 myoblast cell line, primary myoblasts, and primary satellite cells. In vivo overexpression of PAK4 or kinase-inactive mutant PAK4^S474A was conducted in skeletal muscle to examine PAK4's kinase-dependent effect on muscle regeneration. The regeneration process was evaluated by determining the number and size of multinucleated myofibres and expression patterns of myogenin and eMyHC. To explore whether PAK4 inhibition improves muscle regeneration, mice were injected intramuscularly with siRNA that targeted PAK4 or orally administered with a chemical inhibitor of PAK4.

RESULTS

p21-activated kinase 4 was highly expressed during the myoblast stage, but expression gradually and substantially decreased as myoblasts differentiated into myotubes. PAK4 overexpression, but not kinase-inactive mutant PAK4^S474A overexpression, significantly impeded myoblast fusion and MyHC-positive myotube formation in C2C12 cells, primary myoblasts, and satellite cells (P < 0.01). Conversely, PAK4 silencing led to an 8.7% and a 20.3% increase in the number of multinucleated larger myotubes in C2C12 cells and primary myoblasts. Further, in vivo overexpression of PAK4 by adenovirus injection to mice prior to and after myotoxin-induced injury led to a 52.6% decrease in the number of eMyHC-positive myofibres on Day 5 in tibialis anterior muscles as compared with those injected with control adenoviruses (P < 0.01), while Ad-PAK4^S474A showed comparable muscle regeneration parameters. PAK4-induced repression of muscle regeneration coincided with an increase in phosphatase and tensin homologue (PTEN) expression and a decrease in phosphoinositide 3-kinase-Akt signalling. In contrast, PAK4 silencing reduced PTEN expression in mice. Consistent with these findings, prodrug of PAK4 inhibitor CZh-226 (30 mg/kg) orally administered to mice repressed PTEN expression and accelerated myotube formation. Subsequent mechanistic studies revealed that PAK4 directly phosphorylates PPARγ at S273 to increase its transcription activity, thereby up-regulating PTEN expression. Importantly, an analysis of the Genotype-Tissue Expression database showed a positive correlation between PAK4 and PTEN in human skeletal muscle tissues (P < 0.01).

CONCLUSIONS

p1-activated kinase 4 is a new member of PPARγ kinase, and PAK4 inhibition may have a therapeutic role as an accelerant of muscle regeneration.

CRISPR-SID: Identifying EZH2 as a druggable target for desmoid tumors via in vivo dependency mapping

Cancer precision medicine implies identification of tumor-specific vulnerabilities associated with defined oncogenic pathways. Desmoid tumors are soft-tissue neoplasms strictly driven by Wnt signaling network hyperactivation. Despite this clearly defined genetic etiology and the strict and unique implication of the Wnt/β-catenin pathway, no specific molecular targets for these tumors have been identified. To address this caveat, we developed fast, efficient, and penetrant genetic Xenopus tropicalis desmoid tumor models to identify and characterize drug targets. We used multiplexed CRISPR/Cas9 genome editing in these models to simultaneously target a tumor suppressor gene (apc) and candidate dependency genes. Our methodology CRISPR/Cas9 selection-mediated identification of dependencies (CRISPR-SID) uses calculated deviations between experimentally observed gene editing outcomes and deep-learning-predicted double-strand break repair patterns to identify genes under negative selection during tumorigenesis. This revealed EZH2 and SUZ12, both encoding polycomb repressive complex 2 components, and the transcription factor CREB3L1 as genetic dependencies for desmoid tumors. In vivo EZH2 inhibition by Tazemetostat induced partial regression of established autochthonous tumors. In vitro models of patient desmoid tumor cells revealed a direct effect of Tazemetostat on Wnt pathway activity. CRISPR-SID represents a potent approach for in vivo mapping of tumor vulnerabilities and drug target identification.

Administration of a selective retinoic acid receptor-γ agonist improves neuromuscular strength in a rodent model of volumetric muscle loss

PURPOSE

Volumetric muscle loss is a uniquely challenging pathology that results in irrecoverable functional deficits. Furthermore, a breakthrough drug or bioactive factor has yet to be established that adequately improves repair of these severe skeletal muscle injuries. This study sought to assess the ability of an orally administered selective retinoic acid receptor-γ agonist, palovarotene, to improve recovery of neuromuscular strength in a rat model of volumetric muscle loss.

METHODS

An irrecoverable, full thickness defect was created in the tibialis anterior muscle of Lewis rats and animals were survived for 4 weeks. Functional recovery of the tibialis anterior muscle was assessed in vivo via neural stimulation and determination of peak isometric torque. Histological staining was performed to qualitatively assess fibrous scarring of the defect site.

RESULTS

Treatment with the selective retinoic acid receptor-γ agonist, palovarotene, resulted in a 38% improvement of peak isometric torque in volumetric muscle loss affected limbs after 4 weeks of healing compared to untreated controls. Additionally, preliminary histological assessment suggests that oral administration of palovarotene reduced fibrous scarring at the defect site.

CONCLUSIONS

These results highlight the potential role of selective retinoic acid receptor-γ agonists in the design of regenerative medicine platforms to maximize skeletal muscle healing. Additional studies are needed to further elucidate cellular responses, optimize therapeutic delivery, and characterize synergistic potential with adjunct therapies.

Enhancement of myogenic potential of muscle progenitor cells and muscle healing during pregnancy

The decline of muscle regenerative potential with age has been attributed to a diminished responsiveness of muscle progenitor cells (MPCs). Heterochronic parabiosis has been used as a model to study the effects of aging on stem cells and their niches. These studies have demonstrated that, by exposing old mice to a young systemic environment, aged progenitor cells can be rejuvenated. One interesting idea is that pregnancy represents a unique biological model of a naturally shared circulatory system between developing and mature organisms. To test this hypothesis, we evaluated the muscle regeneration potential of pregnant mice using a cardiotoxin (CTX) injury mouse model. Our results indicate that the pregnant mice demonstrate accelerated muscle healing compared to nonpregnant control mice following muscle injury based on improved muscle histology, superior muscle regeneration, and a reduction in inflammation and necrosis. Additionally, we found that MPCs isolated from pregnant mice display a significant improvement of myogenic differentiation capacity in vitro and muscle regeneration in vivo when compared to the MPCs from nonpregnant mice. Furthermore, MPCs from nonpregnant mice display enhanced myogenic capacity when cultured in the presence of serum obtained from pregnant mice. Our proteomics data from these studies provides potential therapeutic targets to enhance the myogenic potential of progenitor cells and muscle repair.

Striated Preferentially Expressed Protein Kinase (SPEG) in Muscle Development, Function, and Disease

Mutations in striated preferentially expressed protein kinase (SPEG), a member of the myosin light chain kinase protein family, are associated with centronuclear myopathy (CNM), cardiomyopathy, or a combination of both. Burgeoning evidence suggests that SPEG plays critical roles in the development, maintenance, and function of skeletal and cardiac muscles. Here we review the genotype-phenotype relationships and the molecular mechanisms of SPEG-related diseases. This review will focus on the progress made toward characterizing SPEG and its interacting partners, and its multifaceted functions in muscle regeneration, triad development and maintenance, and excitation-contraction coupling. We will also discuss future directions that are yet to be investigated including understanding of its tissue-specific roles, finding additional interacting proteins and their relationships. Understanding the basic mechanisms by which SPEG regulates muscle development and function will provide critical insights into these essential processes and help identify therapeutic targets in SPEG-related disorders.

Regulation of muscle stem cell function

Regeneration of skeletal muscle is a finely tuned process which is depending on muscle stem cells, a population of stem cells in skeletal muscle which is also termed satellite cells. Muscle stem cells are a prerequisite for regeneration of skeletal muscle. Of note, the muscle stem cell population is heterogeneous and subpopulations can be identified depending on gene expression or phenotypic traits. However, all muscle stem cells express the transcription factor Pax7 and their functionality is tightly controlled by intrinsic signaling pathways and extrinsic signals. The latter ones include signals form the stem cell niche as well as circulating factors such as growth factors and hormones. Among them are Wnt proteins, growth factors like IGF-1 or FGF-2 and hormones such as thyroid hormones and the anti-aging hormone Klotho. A highly orchestrated interplay between those factors and muscle stem cells is important for their full functionality and ultimately regeneration of skeletal muscle as outlined here.

TGFβ signaling curbs cell fusion and muscle regeneration

Muscle cell fusion is a multistep process involving cell migration, adhesion, membrane remodeling and actin-nucleation pathways to generate multinucleated myotubes. However, molecular brakes restraining cell-cell fusion events have remained elusive. Here we show that transforming growth factor beta (TGFβ) pathway is active in adult muscle cells throughout fusion. We find TGFβ signaling reduces cell fusion, regardless of the cells' ability to move and establish cell-cell contacts. In contrast, inhibition of TGFβ signaling enhances cell fusion and promotes branching between myotubes in mouse and human. Exogenous addition of TGFβ protein in vivo during muscle regeneration results in a loss of muscle function while inhibition of TGFβR2 induces the formation of giant myofibers. Transcriptome analyses and functional assays reveal that TGFβ controls the expression of actin-related genes to reduce cell spreading. TGFβ signaling is therefore requisite to limit mammalian myoblast fusion, determining myonuclei numbers and myofiber size.

Epigenetic regulation of Wnt7b expression by the cis-acting long noncoding RNA Lnc-Rewind in muscle stem cells

Skeletal muscle possesses an outstanding capacity to regenerate upon injury due to the adult muscle stem cell (MuSC) activity. This ability requires the proper balance between MuSC expansion and differentiation, which is critical for muscle homeostasis and contributes, if deregulated, to muscle diseases. Here, we functionally characterize a novel chromatin-associated long noncoding RNA (lncRNA), Lnc-Rewind, which is expressed in murine MuSCs and conserved in human. We find that, in mouse, Lnc-Rewind acts as an epigenetic regulator of MuSC proliferation and expansion by influencing the expression of skeletal muscle genes and several components of the WNT (Wingless-INT) signalling pathway. Among them, we identified the nearby Wnt7b gene as a direct Lnc-Rewind target. We show that Lnc-Rewind interacts with the G9a histone lysine methyltransferase and mediates the in cis repression of Wnt7b by H3K9me2 deposition. Overall, these findings provide novel insights into the epigenetic regulation of adult muscle stem cells fate by lncRNAs.

SIX1 and SIX4 homeoproteins regulate PAX7+ progenitor cell properties during fetal epaxial myogenesis

Pax7 expression marks stem cells in developing skeletal muscles and adult satellite cells during homeostasis and muscle regeneration. The genetic determinants that control the entrance into the myogenic program and the appearance of PAX7+ cells during embryogenesis are poorly understood. SIX homeoproteins are encoded by the sine oculis-related homeobox Six1-Six6 genes in vertebrates. Six1, Six2, Six4 and Six5 are expressed in the muscle lineage. Here, we tested the hypothesis that Six1 and Six4 could participate in the genesis of myogenic stem cells. We show that fewer PAX7+ cells occupy a satellite cell position between the myofiber and its associated basal lamina in Six1 and Six4 knockout mice (s1s4KO) at E18. However, PAX7+ cells are detected in remaining muscle masses present in the epaxial region of the double mutant embryos and are able to divide and contribute to muscle growth. To further characterize the properties of s1s4KO PAX7+ cells, we analyzed their transcriptome and tested their properties after transplantation in adult regenerating tibialis anterior muscle. Mutant stem cells contribute to hypotrophic myofibers that are not innervated but retain the ability to self-renew.

Tissue Stem Cells: Architects of Their Niches

Stem cells (SCs) maintain tissue homeostasis and repair wounds. Despite marked variation in tissue architecture and regenerative demands, SCs often follow similar paradigms in communicating with their microenvironmental "niche" to transition between quiescent and regenerative states. Here we use skin epithelium and skeletal muscle-among the most highly-stressed tissues in our body-to highlight similarities and differences in niche constituents and how SCs mediate natural tissue rejuvenation and perform regenerative acts prompted by injuries. We discuss how these communication networks break down during aging and how understanding tissue SCs has led to major advances in regenerative medicine.

Transcriptomic Profile of Primary Culture of Skeletal Muscle Cells Isolated from Semitendinosus Muscle of Beef and Dairy Bulls

The aim of the study was to identify differences in the transcriptomic profiles of primary muscle cell cultures derived from the semitendinosus muscle of bulls of beef breeds (Limousin (LIM) and Hereford (HER)) and a dairy breed (Holstein-Friesian (HF)) (n = 4 for each breed). Finding a common expression pattern for proliferating cells may point to such an early orientation of the cattle beef phenotype at the transcriptome level of unfused myogenic cells. To check this hypothesis, microarray analyses were performed. The analysis revealed 825 upregulated and 1300 downregulated transcripts similar in both beef breeds (LIM and HER) and significantly different when compared with the dairy breed (HF) used as a reference. Ontological analyses showed that the largest group of genes were involved in muscle organ development. Muscle cells of beef breeds showed higher expression of genes involved in myogenesis (including erbb-3, myf5, myog, des, igf-1, tgfb2) and those encoding proteins comprising the contractile apparatus (acta1, actc1, myh3, myh11, myl1, myl2, myl4, tpm1, tnnt2, tnnc1). The obtained results confirmed our hypothesis that the expression profile of several groups of genes is common in beef breeds at the level of proliferating satellite cells but differs from that observed in typical dairy breeds.

WISP1 promotes bovine MDSC differentiation via recruitment of ANXA1 for the regulation of the TGF-β signalling pathway

Skeletal muscle is one of the most important tissues of the human body necessary for sporting activities. The differentiation of muscle-derived satellite cells (MDSCs) plays an important role in the development and regeneration of skeletal muscles. Similarly, the Wnt/β-catenin signalling pathway plays an important role in the process of muscle differentiation. Wnt1-inducible signalling pathway protein-1 (WISP1), a downstream protein of the Wnt/β-catenin signalling pathway and a member of the CCN family that also plays an important role in the differentiation process, and its expression increase during the differentiation of bovine MDSCs. However, its role in MDSC differentiation is poorly understood. Therefore, we investigated the mechanisms regulating this process via Western blot and immunofluorescence staining. Immunoprecipitation and mass spectrometry detected annexin A1 (ANXA1), a protein that interacts with WISP1. To determine whether WISP1 influences TGF-β signalling and differentiation independently of ANXA1, the latter was knocked down, while WISP1 was activated. WISP1 expression increased significantly during bovine MDSC differentiation. However, WISP1 did not affect the TGF-β signalling pathway protein marker when ANXA1 was inhibited. Taken together, WISP1 regulates the TGF-β signalling pathway through ANXA1 recruitment, thereby promoting bovine MDSC differentiation, suggesting the Wnt/β-catenin signalling pathway as another target to promote cell differentiation.

From gut to glutes: The critical role of niche signals in the maintenance and renewal of adult stem cells

Stem cell behavior is tightly regulated by spatiotemporal signaling from the niche, which is a four-dimensional microenvironment that can instruct stem cells to remain quiescent, self-renew, proliferate, or differentiate. In this review, we discuss recent advances in understanding the signaling cues provided by the stem cell niche in two contrasting adult tissues, the rapidly cycling intestinal epithelium and the slowly renewing skeletal muscle. Drawing comparisons between these two systems, we discuss the effects of niche-derived growth factors and signaling molecules, metabolic cues, the extracellular matrix and biomechanical cues, and immune signals on stem cells. We also discuss the influence of the niche in defining stem cell identity and function in both normal and pathophysiologic states.

A Cdh1-FoxM1-Apc axis controls muscle development and regeneration

Forkhead box M1 (FoxM1) transcriptional factor has a principal role in regulating cell proliferation, self-renewal, and tumorigenesis. However, whether FoxM1 regulates endogenous muscle development and regeneration remains unclear. Here we found that loss of FoxM1 in muscle satellite cells (SCs) resulted in muscle atrophy and defective muscle regeneration. FoxM1 functioned as a direct transcription activator of adenomatous polyposis coli (Apc), preventing hyperactivation of wnt/β-catenin signaling during muscle regeneration. FoxM1 overexpression in SCs promoted myogenesis but impaired muscle regeneration as a result of spontaneous activation and exhaustion of SCs by transcriptional regulation of Cyclin B1 (Ccnb1). The E3 ubiquitin ligase Cdh1 (also termed Fzr1) was required for FoxM1 ubiquitylation and subsequent degradation. Loss of Cdh1 promoted quiescent SCs to enter into the cell cycle and the SC pool was depleted by serial muscle injuries. Haploinsufficiency of FoxM1 ameliorated muscle regeneration of Cdh1 knock-out mice. These data demonstrate that the Cdh1–FoxM1–Apc axis functions as a key regulator of muscle development and regeneration.

Overcoming resistance to anabolic SARM therapy in experimental cancer cachexia with an HDAC inhibitor

No approved therapy exists for cancer-associated cachexia. The colon-26 mouse model of cancer cachexia mimics recent late-stage clinical failures of anabolic anti-cachexia therapy and was unresponsive to anabolic doses of diverse androgens, including the selective androgen receptor modulator (SARM) GTx-024. The histone deacetylase inhibitor (HDACi) AR-42 exhibited anti-cachectic activity in this model. We explored combined SARM/AR-42 therapy as an improved anti-cachectic treatment paradigm. A reduced dose of AR-42 provided limited anti-cachectic benefits, but, in combination with GTx-024, significantly improved body weight, hindlimb muscle mass, and grip strength versus controls. AR-42 suppressed the IL-6/GP130/STAT3 signaling axis in muscle without impacting circulating cytokines. GTx-024-mediated β-catenin target gene regulation was apparent in cachectic mice only when combined with AR-42. Our data suggest cachectic signaling in this model involves catabolic signaling insensitive to anabolic GTx-024 therapy and a blockade of GTx-024-mediated anabolic signaling. AR-42 mitigates catabolic gene activation and restores anabolic responsiveness to GTx-024. Combining GTx-024, a clinically established anabolic therapy, with AR-42, a clinically evaluated HDACi, represents a promising approach to improve anabolic response in cachectic patients.

Metabolic reprogramming of fibro/adipogenic progenitors facilitates muscle regeneration

High-fat diet ameliorates muscle dystrophic phenotype by promoting the FAP-dependent myogenesis of satellite cells.

In Duchenne muscular dystrophy (DMD), the absence of the dystrophin protein causes a variety of poorly understood secondary effects. Notably, muscle fibers of dystrophic individuals are characterized by mitochondrial dysfunctions, as revealed by a reduced ATP production rate and by defective oxidative phosphorylation. Here, we show that in a mouse model of DMD (mdx), fibro/adipogenic progenitors (FAPs) are characterized by a dysfunctional mitochondrial metabolism which correlates with increased adipogenic potential. Using high-sensitivity mass spectrometry–based proteomics, we report that a short-term high-fat diet (HFD) reprograms dystrophic FAP metabolism in vivo. By combining our proteomic dataset with a literature-derived signaling network, we revealed that HFD modulates the β-catenin–follistatin axis. These changes are accompanied by significant amelioration of the histological phenotype in dystrophic mice. Transplantation of purified FAPs from HFD-fed mice into the muscles of dystrophic recipients demonstrates that modulation of FAP metabolism can be functional to ameliorate the dystrophic phenotype. Our study supports metabolic reprogramming of muscle interstitial progenitor cells as a novel approach to alleviate some of the adverse outcomes of DMD.

Myogenic differentiation of human amniotic mesenchymal cells and its tissue repair capacity on volumetric muscle loss

Stem cell-based tissue engineering therapy is the most promising method for treating volumetric muscle loss. Human amniotic mesenchymal cells possess characteristics similar to those of embryonic stem cells. In this study, we verified the stem cell characteristics of human amniotic mesenchymal cells by the flow cytometry analysis, and osteogenic and adipogenic differentiation. Through induction with the DNA demethylating agent 5-azacytidine, human amniotic mesenchymal cells can undergo myogenic differentiation and express skeletal muscle cell-specific markers such as desmin and MyoD. The Wnt/β-catenin signaling pathway also plays an important role. After 5-azacytidine-induced human amniotic mesenchymal cells were implanted into rat tibialis anterior muscle with volumetric muscle loss, we observed increased angiogenesis and improved local tissue repair. We believe that human amniotic mesenchymal cells can serve as a potential source of cells for skeletal muscle tissue engineering.

Insights into intestinal regeneration signaling mechanisms

The cellular mechanisms underlying the amazing ability of sea cucumbers to regenerate their autotomized intestines have been widely described by us and others. However, the signaling pathways that control these mechanisms are unknown. Previous studies have shown that Wnt homologs are upregulated during early intestinal regenerative stages, suggesting that the Wnt/β-catenin pathway is active during this process. Here, we used small molecules, putative disruptors of the Wnt pathway, to determine the potential role of the canonical Wnt pathway on intestine regeneration in the sea cucumber Holothuria glaberrima. We evaluated their effects in vivo by using histological analyses for cell dedifferentiation, cell proliferation and apoptosis. We found that iCRT14, an alleged Wnt pathway inhibitor, decreased the size of the regenerating intestine, while LiCl, a presumed Wnt pathway activator, increased its size. The possible cellular mechanisms by which signaling pathway disruptors affect the gut rudiment size were further studied in vitro, using cultures of tissue explants and additional pharmacological agents. Among the tested signaling activators, those that act through GSK-3 inhibition, LiCl, 1-Azakenpaullone, and CHIR99021 were found to increase muscle cell dedifferentiation, while the inhibitor iCRT14 blocked cell dedifferentiation. Differently, cell proliferation was reduced by all GSK-3 inhibitors, as well as by iCRT14 and C59, which interferes with Wnt ligand secretion. The in vivo temporal and spatial pattern of β-catenin activity was determined using an antibody against phosphorylated β-catenin and shown to correlate with cell proliferative activity. In vitro treatment using C59 decreased the number of cells immunostained for nuclear phosphorylated β-catenin. Our results showed that the cell dedifferentiation observed during intestinal regeneration can be decoupled from the cell proliferation event and that these cellular processes can be modulated by particular signaling pathway inhibitors and activators. These results open the door for future studies where the cellular signaling pathways involved at each regeneration stage can be determined.

(Pro)renin receptor accelerates development of sarcopenia via activation of Wnt/YAP signaling axis

To extend life expectancy and ensure healthy aging, it is crucial to prevent and minimize age‐induced skeletal muscle atrophy, also known as sarcopenia. However, the disease's molecular mechanism remains unclear. The age‐related Wnt/β‐catenin signaling pathway has been recently shown to be activated by the (pro)renin receptor ((P)RR). We report here that (P)RR expression was increased in the atrophied skeletal muscles of aged mice and humans. Therefore, we developed a gain‐of‐function model of age‐related sarcopenia via transgenic expression of (P)RR under control of the CAG promoter. Consistent with our hypothesis, (P)RR‐Tg mice died early and exhibited muscle atrophy with histological features of sarcopenia. Moreover, Wnt/β‐catenin signaling was activated and the regenerative capacity of muscle progenitor cells after cardiotoxin injury was impaired due to cell fusion failure in (P)RR‐Tg mice. In vitro forced expression of (P)RR protein in C2C12 myoblast cells suppressed myotube formation by activating Wnt/β‐catenin signaling. Administration of Dickkopf‐related protein 1, an inhibitor of Wnt/β‐catenin signaling, and anti‐(P)RR neutralizing antibody, which inhibits binding of (P)RR to the Wnt receptor, significantly improved sarcopenia in (P)RR‐Tg mice. Furthermore, the use of anti‐(P)RR neutralizing antibodies significantly improved the regenerative ability of skeletal muscle in aged mice. Finally, we show that Yes‐associated protein (YAP) signaling, which is coordinately regulated by Wnt/β‐catenin, contributed to the development of (P)RR‐induced sarcopenia. The present study demonstrates the use of (P)RR‐Tg mice as a novel sarcopenia model, and shows that (P)RR‐Wnt‐YAP signaling plays a pivotal role in the pathogenesis of this disease.