Requirement for serum response factor for skeletal muscle growth and maturation revealed by tissue-specific gene deletion in mice

Loss of Skeletal Muscle Inositol Polyphosphate Multikinase Disrupts Glucose Regulation and Limits Exercise Capacity

Inositol phosphates are critical signaling messengers involved in a wide range of biological pathways, and inositol polyphosphate multikinase (IPMK) functions as a rate-limiting enzyme for inositol polyphosphate metabolism. IPMK has been implicated in cellular metabolism, but its function at the systemic level is still poorly understood. Since skeletal muscle is a major contributor to energy homeostasis, we have developed a mouse model in which skeletal muscle IPMK is specifically deleted and examined how a loss of IPMK affects whole-body metabolism. Here, we report that skeletal-muscle-specific IPMK knockout mice exhibited a ~12% increase in body weight compared to WT controls (p < 0.05). These mice also showed a significantly impaired glucose tolerance, as indicated by their ~50% higher blood glucose levels during GTT. Additionally, exercise capacity was reduced by ~45% in IPMK-MKO mice, demonstrating a decline in endurance. Moreover, these metabolic alterations were accompanied by a 2.5-fold increase in skeletal muscle triglyceride accumulation, suggesting impaired lipid metabolism. Further analysis revealed that IPMK-deficient myocytes exhibited 30% lower β-oxidation rates. Thus, our results suggest that IPMK mediates whole-body metabolism by regulating muscle metabolism and may be potentially targeted for the treatment of metabolic syndromes.

SOX6 AU controls myogenesis by cis-modulation of SOX6 in cattle

Long non-coding RNAs (lncRNAs) have been shown to be involved in the regulation of skeletal muscle development through multiple mechanisms. The present study revealed that the lncRNA SOX6 AU (SRY-box transcription factor 6 antisense upstream) is reverse transcribed from upstream of the bovine sex-determining region Y (SRY)-related high-mobility-group box 6 (SOX6) gene. SOX6 AU was significantly differentially expressed in muscle tissue among different developmental stages in Xianan cattle. Subsequently, knockdown and overexpression experiments discovered that SOX6 AU promoted primary skeletal muscle cells proliferation, apoptosis, and differentiation in bovine. The overexpression of SOX6 AU in bovine primary skeletal muscle cells resulted in 483 differentially expressed genes (DEGs), including 224 upregulated DEGs and 259 downregulated DEGs. GO functional annotation analysis showed that muscle development-related biological processes such as muscle structure development and muscle cell proliferation were significantly enriched. KEGG pathway analysis revealed that the PI3K/AKT and MAPK signaling pathways were important pathways for DEG enrichment. Notably, we found that SOX6 AU inhibited the mRNA and protein expression levels of the SOX6 gene. Moreover, knockdown of the SOX6 gene promoted the proliferation and apoptosis of bovine primary skeletal muscle cells. Finally, we showed that SOX6 AU promoted the proliferation and apoptosis of bovine primary skeletal muscle cells by cis-modulation of SOX6 in cattle. This work illustrates our discovery of the molecular mechanisms underlying the regulation of SOX6 AU in the development of beef.

Suppressing PDGFRβ Signaling Enhances Myocyte Fusion to Promote Skeletal Muscle Regeneration

Muscle cell fusion is critical for forming and maintaining multinucleated myotubes during skeletal muscle development and regeneration. However, the molecular mechanisms directing cell-cell fusion are not fully understood. Here, we identify platelet-derived growth factor receptor beta (PDGFRβ) signaling as a key modulator of myocyte fusion in adult muscle cells. Our findings demonstrate that genetic deletion of Pdgfrβ enhances muscle regeneration and increases myofiber size, whereas PDGFRβ activation impairs muscle repair. Inhibition of PDGFRβ activity promotes myonuclear accretion in both mouse and human myotubes, whereas PDGFRβ activation stalls myotube development by preventing cell spreading to limit fusion potential. Transcriptomics analysis show that PDGFRβ signaling cooperates with TGFβ signaling to direct myocyte size and fusion. Mechanistically, PDGFRβ signaling requires STAT1 activation, and blocking STAT1 phosphorylation enhances myofiber repair and size during regeneration. Collectively, PDGFRβ signaling acts as a regenerative checkpoint and represents a potential clinical target to rapidly boost skeletal muscle repair.

The G4 resolvase Dhx36 modulates cardiomyocyte differentiation and ventricular conduction system development

Extensive genetic studies have elucidated cardiomyocyte differentiation and associated gene networks using single-cell RNA-seq, yet the intricate transcriptional mechanisms governing cardiac conduction system (CCS) development and working cardiomyocyte differentiation remain largely unexplored. Here we show that mice deleted for Dhx36 (encoding the Dhx36 helicase) in the embryonic or neonatal heart develop overt dilated cardiomyopathy, surface ECG alterations related to cardiac impulse propagation, and (in the embryonic heart) a lack of a ventricular conduction system (VCS). Heart snRNA-seq and snATAC-seq reveal the role of Dhx36 in CCS development and in the differentiation of working cardiomyocytes. Dhx36 deficiency directly influences cardiomyocyte gene networks by disrupting the resolution of promoter G-quadruplexes in key cardiac genes, impacting cardiomyocyte differentiation and CCS morphogenesis, and ultimately leading to dilated cardiomyopathy and atrioventricular block. These findings further identify crucial genes and pathways that regulate the development and function of the VCS/Purkinje fiber (PF) network.

Zfp697 is an RNA-binding protein that regulates skeletal muscle inflammation and remodeling

Skeletal muscle atrophy is a morbidity and mortality risk factor that happens with disuse, chronic disease, and aging. The tissue remodeling that happens during recovery from atrophy or injury involves changes in different cell types such as muscle fibers, and satellite and immune cells. Here, we show that the previously uncharacterized gene and protein Zfp697 is a damage-induced regulator of muscle remodeling. Zfp697/ZNF697 expression is transiently elevated during recovery from muscle atrophy or injury in mice and humans. Sustained Zfp697 expression in mouse muscle leads to a gene expression signature of chemokine secretion, immune cell recruitment, and extracellular matrix remodeling. Notably, although Zfp697 is expressed in several cell types in skeletal muscle, myofiber-specific Zfp697 genetic ablation in mice is sufficient to hinder the inflammatory and regenerative response to muscle injury, compromising functional recovery. We show that Zfp697 is an essential mediator of the interferon gamma response in muscle cells and that it functions primarily as an RNA-interacting protein, with a very high number of miRNA targets. This work identifies Zfp697 as an integrator of cell-cell communication necessary for tissue remodeling and regeneration.

The TAS1R2 G-protein-coupled receptor is an ambient glucose sensor in skeletal muscle that regulates NAD homeostasis and mitochondrial capacity

The bioavailability of nicotinamide adenine dinucleotide (NAD) is vital for skeletal muscle health, yet the mechanisms or signals regulating NAD homeostasis remain unclear. Here, we uncover a pathway connecting peripheral glucose sensing to the modulation of muscle NAD through TAS1R2, the sugar-sensing G protein-coupled receptor (GPCR) initially identified in taste perception. Muscle TAS1R2 receptor stimulation by glucose and other agonists induces ERK1/2-dependent phosphorylation and activation of poly(ADP-ribose) polymerase1 (PARP1), a major NAD consumer in skeletal muscle. Consequently, muscle-specific deletion of TAS1R2 (mKO) in male mice suppresses PARP1 activity, elevating NAD levels and enhancing mitochondrial capacity and running endurance. Plasma glucose levels negatively correlate with muscle NAD, and TAS1R2 receptor deficiency enhances NAD responses across the glycemic range, implicating TAS1R2 as a peripheral energy surveyor. These findings underscore the role of GPCR signaling in NAD regulation and propose TAS1R2 as a potential therapeutic target for maintaining muscle health.

Mustn1 is a smooth muscle cell-secreted microprotein that modulates skeletal muscle extracellular matrix composition

OBJECTIVE

Skeletal muscle plasticity and remodeling are critical for adapting tissue function to use, disuse, and regeneration. The aim of this study was to identify genes and molecular pathways that regulate the transition from atrophy to compensatory hypertrophy or recovery from injury. Here, we have used a mouse model of hindlimb unloading and reloading, which causes skeletal muscle atrophy, and compensatory regeneration and hypertrophy, respectively.

METHODS

We analyzed mouse skeletal muscle at the transition from hindlimb unloading to reloading for changes in transcriptome and extracellular fluid proteome. We then used qRT-PCR, immunohistochemistry, and bulk and single-cell RNA sequencing data to determine Mustn1 gene and protein expression, including changes in gene expression in mouse and human skeletal muscle with different challenges such as exercise and muscle injury. We generated Mustn1-deficient genetic mouse models and characterized them in vivo and ex vivo with regard to muscle function and whole-body metabolism. We isolated smooth muscle cells and functionally characterized them, and performed transcriptomics and proteomics analysis of skeletal muscle and aorta of Mustn1-deficient mice.

RESULTS

We show that Mustn1 (Musculoskeletal embryonic nuclear protein 1, also known as Mustang) is highly expressed in skeletal muscle during the early stages of hindlimb reloading. Mustn1 expression is transiently elevated in mouse and human skeletal muscle in response to intense exercise, resistance exercise, or injury. We find that Mustn1 expression is highest in smooth muscle-rich tissues, followed by skeletal muscle fibers. Muscle from heterozygous Mustn1-deficient mice exhibit differences in gene expression related to extracellular matrix and cell adhesion, compared to wild-type littermates. Mustn1-deficient mice have normal muscle and aorta function and whole-body glucose metabolism. We show that Mustn1 is secreted from smooth muscle cells, and that it is present in arterioles of the muscle microvasculature and in muscle extracellular fluid, particularly during the hindlimb reloading phase. Proteomics analysis of muscle from Mustn1-deficient mice confirms differences in extracellular matrix composition, and female mice display higher collagen content after chemically induced muscle injury compared to wild-type littermates.

CONCLUSIONS

We show that, in addition to its previously reported intracellular localization, Mustn1 is a microprotein secreted from smooth muscle cells into the muscle extracellular space. We explore its role in muscle ECM deposition and remodeling in homeostasis and upon muscle injury. The role of Mustn1 in fibrosis and immune infiltration upon muscle injury and dystrophies remains to be investigated, as does its potential for therapeutic interventions.

YTHDF2 governs muscle size through a targeted modulation of proteostasis

The regulation of proteostasis is fundamental for maintenance of muscle mass and function. Activation of the TGF-β pathway drives wasting and premature aging by favoring the proteasomal degradation of structural muscle proteins. Yet, how this critical post-translational mechanism is kept in check to preserve muscle health remains unclear. Here, we reveal the molecular link between the post-transcriptional regulation of m6A-modified mRNA and the modulation of SMAD-dependent TGF-β signaling. We show that the m6A-binding protein YTHDF2 is essential to determining postnatal muscle size. Indeed, muscle-specific genetic deletion of YTHDF2 impairs skeletal muscle growth and abrogates the response to hypertrophic stimuli. We report that YTHDF2 controls the mRNA stability of the ubiquitin ligase ASB2 with consequences on anti-growth gene program activation through SMAD3. Our study identifies a post-transcriptional to post-translational mechanism for the coordination of gene expression in muscle.

Post-transcriptional regulation of myogenic transcription factors during muscle development and pathogenesis

The transcriptional regulation of skeletal muscle (SKM) development (myogenesis) has been documented for over 3 decades and served as a paradigm for tissue-specific cell type determination and differentiation. Myogenic stem cells (MuSC) in embryos and adult SKM are regulated by the transcription factors Pax3 and Pax7 for their stem cell characteristics, while their lineage determination and terminal differentiation are both dictated by the myogenic regulatory factors (MRF) that comprise Mrf4, Myf5, Myogenin, and MyoD. The myocyte enhancer factor Mef2c is activated by MRF during terminal differentiation and collaborates with them to promote myoblast fusion and differentiation. Recent studies have found critical regulation of these myogenic transcription factors at mRNA level, including subcellular localization, stability, and translational regulation. Therefore, the regulation of Pax3/7, MRFs and Mef2c mRNAs by RNA-binding factors and non-coding RNAs (ncRNA), including microRNAs and long non-coding RNAs (lncRNA), will be the focus of this review and the impact of this regulation on myogenesis will be further addressed. Interestingly, the stem cell characteristics of MuSC has been found to be critically regulated by ncRNAs, implying the involvement of ncRNAs in SKM homeostasis and regeneration. Current studies have further identified that some ncRNAs are implicated in the etiology of some SKM diseases and can serve as valuable tools/indicators for prediction of prognosis. The roles of ncRNAs in the MuSC biology and SKM disease etiology will also be discussed in this review.

WAVE2 Is a Vital Regulator in Myogenic Differentiation of Progenitor Cells through the Mechanosensitive MRTFA-SRF Axis

Skeletal myogenesis is an intricate process involving the differentiation of progenitor cells into myofibers, which is regulated by actin cytoskeletal dynamics and myogenic transcription factors. Although recent studies have demonstrated the pivotal roles of actin-binding proteins (ABPs) as mechanosensors and signal transducers, the biological significance of WAVE2 (Wiskott-Aldrich syndrome protein family member 2), an ABP essential for actin polymerization, in myogenic differentiation of progenitor cells has not been investigated. Our study provides important insights into the regulatory roles played by WAVE2 in the myocardin-related transcription factor A (MRTFA)-serum response factor (SRF) signaling axis and differentiation of myoblasts. We demonstrate that WAVE2 expression is induced during myogenic differentiation and plays a pivotal role in actin cytoskeletal remodeling in C2C12 myoblasts. Knockdown of WAVE2 in C2C12 cells reduced filamentous actin levels, increased globular actin accumulation, and impaired the nuclear translocation of MRTFA. Furthermore, WAVE2 depletion in myoblasts inhibited the expression and transcriptional activity of SRF and suppressed cell proliferation in myoblasts. Consequently, WAVE2 knockdown suppressed myogenic regulatory factors (i.e., MyoD, MyoG, and SMYD1) expressions, thereby hindering the differentiation of myoblasts. Thus, this study suggests that WAVE2 is essential for myogenic differentiation of progenitor cells by modulating the mechanosensitive MRTFA-SRF axis.

Lysine methylation signaling in skeletal muscle biology: from myogenesis to clinical insights

Lysine methylation signaling is well studied for its key roles in the regulation of transcription states through modifications on histone proteins. While histone lysine methylation has been extensively studied, recent discoveries of lysine methylation on thousands of non-histone proteins has broadened our appreciation for this small chemical modification in the regulation of protein function. In this review, we highlight the significance of histone and non-histone lysine methylation signaling in skeletal muscle biology, spanning development, maintenance, regeneration, and disease progression. Furthermore, we discuss potential future implications for its roles in skeletal muscle biology as well as clinical applications for the treatment of skeletal muscle-related diseases.

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

Serum Response Factor Reduces Gene Expression Noise and Confers Cell State Stability

The role of serum response factor (Srf), a central mediator of actin dynamics and mechanical signaling, in cell identity regulation is debated to be either a stabilizer or a destabilizer. We investigated the role of Srf in cell fate stability using mouse pluripotent stem cells. Despite the fact that serum-containing cultures yield heterogeneous gene expression, deletion of Srf in mouse pluripotent stem cells leads to further exacerbated cell state heterogeneity. The exaggerated heterogeneity is detectible not only as increased lineage priming but also as the developmentally earlier 2C-like cell state. Thus, pluripotent cells explore more variety of cellular states in both directions of development surrounding naïve pluripotency, a behavior that is constrained by Srf. These results support that Srf functions as a cell state stabilizer, providing rationale for its functional modulation in cell fate intervention and engineering.

Zfp697 is an RNA-binding protein that regulates skeletal muscle inflammation and regeneration

Muscular atrophy is a mortality risk factor that happens with disuse, chronic disease, and aging. Recovery from atrophy requires changes in several cell types including muscle fibers, and satellite and immune cells. Here we show that Zfp697/ZNF697 is a damage-induced regulator of muscle regeneration, during which its expression is transiently elevated. Conversely, sustained Zfp697 expression in mouse muscle leads to a gene expression signature of chemokine secretion, immune cell recruitment, and extracellular matrix remodeling. Myofiber-specific Zfp697 ablation hinders the inflammatory and regenerative response to muscle injury, compromising functional recovery. We uncover Zfp697 as an essential interferon gamma mediator in muscle cells, interacting primarily with ncRNAs such as the pro-regenerative miR-206. In sum, we identify Zfp697 as an integrator of cell-cell communication necessary for tissue regeneration.

Circadian clock control of MRTF/SRF pathway suppresses beige adipocyte thermogenic recruitment

The morphological transformation of adipogenic progenitors into mature adipocytes requires dissolution of actin cytoskeleton with loss of myocardin-related transcription factor (MRTF)/serum response factor (SRF) activity. Circadian clock confers temporal control in adipogenic differentiation, while the actin cytoskeleton-MRTF/SRF signaling transduces extracellular physical niche cues. Here, we define a novel circadian transcriptional control involved in actin cytoskeleton-MRTF/SRF signaling cascade that modulates beige fat thermogenic function. Key components of actin dynamic-MRTF/SRF pathway display circadian regulation in beige fat depot. The core clock regulator, brain and muscle arnt-like 1 (Bmal1), exerts direct transcriptional control of genes within the actin dynamic-MRTF/SRF cascade that impacts actin cytoskeleton organization and SRF activity. Employing beige fat-selective gene-targeting models together with pharmacological rescues, we further demonstrate that Bmal1 inhibits beige adipogenesis and thermogenic capacity in vivo via the MRTF/SRF pathway. Selective ablation of Bmal1 induces beigeing with improved glucose homeostasis, whereas its targeted overexpression attenuates thermogenic induction resulting in obesity. Collectively, our findings identify the clock-MRTF/SRF regulatory axis as an inhibitory mechanism of beige fat thermogenic recruitment with significant contribution to systemic metabolic homeostasis.

SRF/MRTF-A and liver cirrhosis: Pathologic associations

Liver cirrhosis results from prolonged and extensive liver fibrosis in which fibrotic tissues replace functional hepatic cells. Chronic liver disease due to various viral, chemical, or metabolic factors initiates hepatic fibrogenesis. Cirrhosis is associated with multiple clinical complications and a poor patient prognosis; therefore, developing novel antifibrotic therapies to prevent cirrhosis is of high priority. Mounting evidence points to the key role of serum response factor (SRF) and myocardin-related transcription factor (MRTF)-A in the pathogenesis of liver fibrosis. SRF is a transcription factor and MRTF-A is a co-activator of SRF and normally resides in the cytoplasm. Upon the induction of fibrotic pathways, MRTF-A translocates into the nucleus and forms the active SRF/MRTF-A complex, leading to the expression of a multitude of fibrotic proteins and components of extracellular matrix. Silencing or inhibiting MRTF-A impedes hepatic stellate cell transdifferentiation into myofibroblasts and slows down the deposition of extracellular matrix in the liver, making it a potential therapeutic target. Here, we review the recent findings regarding the role of the SRF/MRTF-A complex in liver fibrosis and its therapeutic potential for the management of cirrhosis.

Comparative Genomic Characterization of Insulin-Like Growth Factor Binding Proteins in Cattle and Buffalo

The somatotropic axis consists of genes associated with economic traits like muscle growth and carcass traits in livestock. Insulin-like growth factor binding proteins (IGFBPs) are the major proteins that play a vital role in the somatotropic axis. The present study performed a genome-wide characterization of IGFBP genes in cattle. Genomic sequences of the IGFBP gene family for different mammals (cattle, buffalo, goat, and sheep) were recovered from the NCBI database. Sequence analyses were performed to investigate cattle's genomic variations in the IGFBP gene family. Phylogenetic analysis, gene structure, motif patterns, and conserved domain analysis (CDA) of the IGFBP family revealed the evolutionarily conserved nature of the IGFBP genes in cattle and other studied species. Physicochemical properties of IGFBP proteins in cattle revealed that most of these proteins are unstable, hydrophilic, thermostable, and acidic. Comparative amino acid analysis revealed variations in all protein sequences with single indels in IGFBP3 and IGFBP6. Mutation analysis revealed only one nonsynonymous mutation D212 > E in the IGFBP6 protein of cattle. A total of 245 nuclear hormone receptor (NHRs) sites were detected, including 139 direct repeats (DR), 65 everted repeats (ER), and 41 inverted repeats (IR). Out of 133 transcription factors (TFs), 10 TFs (AHR, AHRARNT, AP4, CMYB, E47, EGR2, GATA, SP1, and SRF) had differential distribution (P value < 0.05) of putative transcriptional binding sites (TFBS) in cattle compared to buffalo. Synteny analysis revealed the conserved nature of genes between cattle and buffalo. Two gene pairs (IGFBP1/IGFBP3 and IGFBP2/IGFBP5) showed tandem duplication events in cattle and buffalo. This study highlights the functional importance of genomic variation in IGFBP genes and necessitates further investigations better to understand the role and mechanisms of IGFBPs in bovines.

MicroRNA expression signature in the striated and smooth adductor muscles of Yesso scallop Patinopecten yessoensis

Increasing evidences point to the potential role of microRNAs (miRNAs) in muscle growth and development in animals. However, knowledge on the identity of miRNAs and their targets in molluscs remains largely unknown. Scallops have one large adductor muscle, composed of fast (striated) and slow (smooth) muscle types, which display great differences in muscle fibers, meat quality, cell types and molecular components. In the present study, we performed a comprehensive investigation of miRNA transcriptomes in fast and slow adductor muscles of Yesso scallop Patinopecten yessoensis. As a result, 47 differentially expressed miRNAs representing ten miRNA families were identified between the striated and smooth adductor muscles. The KEGG enrichment analysis of their target genes were mainly associated with amino acid metabolism, energy metabolism and glycan biosynthesis. The target genes of miR-133 and miR-71 were validated by the dual-luciferase reporter assays and miRNA antagomir treatment in vivo. The identification and functional validation of these different miRNAs in scallops will greatly help our understanding of miRNA regulatory mechanism that achieves the unique muscle phenotypes in scallops. The present findings provide the direct evidences for muscle-specific miRNAs involved in muscle growth and differentiation in molluscs.

Cytoplasmic HDAC4 regulates the membrane repair mechanism in Duchenne muscular dystrophy

BACKGROUND

Histone deacetylase 4 (HDAC4) is a stress-responsive factor that mediates multiple cellular responses. As a member of class IIa HDACs, HDAC4 shuttles between the nucleus and the cytoplasm; however, HDAC4 cytoplasmic functions have never been fully investigated. Duchenne muscular dystrophy (DMD) is a genetic, progressive, incurable disorder, characterized by muscle wasting, which can be treated with the unspecific inhibition of HDACs, despite this approach being only partially effective. More efficient strategies may be proposed for DMD only after the different HDAC members will be characterized.

METHODS

To fully understand HDAC4 functions, we generated dystrophic mice carrying a skeletal muscle-specific deletion of HDAC4 (mdx;KO mice). The progression of muscular dystrophy was characterized in mdx and age-matched mdx;KO mice by means of histological, molecular, and functional analyses. Satellite cells (SCs) from these mice were differentiated in vitro, to identify HDAC4 intrinsic functions influencing the myogenic potential of dystrophic SCs. Gain-of-function experiments revealed the cytoplasmic functions of HDAC4 in mdx;KO muscles.

RESULTS

Histone deacetylase 4 increased in the skeletal muscles of mdx mice (~3-fold; P < 0.05) and of DMD patients (n = 3, males, mean age 13.3 ± 1.5 years), suggesting that HDAC4 has a role in DMD. Its deletion in skeletal muscles importantly worsens the pathological features of DMD, leading to greater muscle fragility and degeneration over time. Additionally, it impairs SC survival, myogenic potential, and muscle regeneration, ultimately compromising muscle function (P < 0.05-0.001). The impaired membrane repair mechanism in muscles and SCs accounts for the mdx;KO phenotype. Indeed, the ectopic expression of Trim72, a major player in the membrane repair mechanism, prevents SC death (~20%; P < 0.01) and increases myogenic fusion (~40%; P < 0.01) in vitro; in vivo it significantly reduces myofibre damage (~10%; P < 0.005) and improves mdx;KO muscle function (P < 0.05). The mdx;KO phenotype is also fully rescued by restoring cytoplasmic levels of HDAC4, both in vitro and in vivo. The protective role of HDAC4 in the cytoplasm of mdx;KO muscles is, in part, independent of its deacetylase activity. HDAC4 expression correlates with Trim72 mRNA levels; furthermore, Trim72 mRNA decays more rapidly (P < 0.01) in mdx;KO muscle cells, compared with mdx ones.

CONCLUSIONS

Histone deacetylase 4 performs crucial functions in the cytoplasm of dystrophic muscles, by mediating the muscle repair response to damage, an important role in ensuring muscle homeostasis, probably by stabilizing Trim72 mRNA. Consequently, the cytoplasmic functions of HDAC4 should be stimulated rather than inhibited in muscular dystrophy treatments, a fact to be considered in future therapeutic approaches.

Cellular and molecular pathways controlling muscle size in response to exercise

From the discovery of ATP and motor proteins to synaptic neurotransmitters and growth factor control of cell differentiation, skeletal muscle has provided an extreme model system in which to understand aspects of tissue function. Muscle is one of the few tissues that can undergo both increase and decrease in size during everyday life. Muscle size depends on its contractile activity, but the precise cellular and molecular pathway(s) by which the activity stimulus influences muscle size and strength remain unclear. Four correlates of muscle contraction could, in theory, regulate muscle growth: nerve-derived signals, cytoplasmic calcium dynamics, the rate of ATP consumption and physical force. Here, we summarise the evidence for and against each stimulus and what is known or remains unclear concerning their molecular signal transduction pathways and cellular effects. Skeletal muscle can grow in three ways, by generation of new syncytial fibres, addition of nuclei from muscle stem cells to existing fibres or increase in cytoplasmic volume/nucleus. Evidence suggests the latter two processes contribute to exercise-induced growth. Fibre growth requires increase in sarcolemmal surface area and cytoplasmic volume at different rates. It has long been known that high-force exercise is a particularly effective growth stimulus, but how this stimulus is sensed and drives coordinated growth that is appropriately scaled across organelles remains a mystery.

Differential regulation of cranial and cardiac neural crest by serum response factor and its cofactors

Serum response factor (SRF) is an essential transcription factor that influences many cellular processes including cell proliferation, migration, and differentiation. SRF directly regulates and is required for immediate early gene (IEG) and actin cytoskeleton-related gene expression. SRF coordinates these competing transcription programs through discrete sets of cofactors, the ternary complex factors (TCFs) and myocardin-related transcription factors (MRTFs). The relative contribution of these two programs to in vivo SRF activity and mutant phenotypes is not fully understood. To study how SRF utilizes its cofactors during development, we generated a knock-in SrfaI allele in mice harboring point mutations that disrupt SRF-MRTF-DNA complex formation but leave SRF-TCF activity unaffected. Homozygous SrfaI/aI mutants die at E10.5 with notable cardiovascular phenotypes, and neural crest conditional mutants succumb at birth to defects of the cardiac outflow tract but display none of the craniofacial phenotypes associated with complete loss of SRF in that lineage. Our studies further support an important role for MRTF mediating SRF function in cardiac neural crest and suggest new mechanisms by which SRF regulates transcription during development.

NLK is required for Ras/ERK/SRF/ELK signaling to tune skeletal muscle development by phosphorylating SRF and antagonizing the SRF/MKL pathway

Serum response factor (SRF) regulates differentiation and proliferation by binding to RhoA-actin-activated MKL or Ras-MAPK-activated ELK transcriptional coactivators, but the molecular mechanisms responsible for SRF regulation remain unclear. Here, we show that Nemo-like kinase (NLK) is required for the promotion of SRF/ELK signaling in human and mouse cells. NLK was found to interact with and phosphorylate SRF at serine residues 101/103, which in turn enhanced the association between SRF and ELK. The enhanced affinity of SRF/ELK antagonized the SRF/MKL pathway and inhibited mouse myoblast differentiation in vitro. In a skeletal muscle-specific Nlk conditional knockout mouse model, forming muscle myofibers underwent hypertrophic growth, resulting in an increased muscle and body mass phenotype. We propose that both phosphorylation of SRF by NLK and phosphorylation of ELKs by MAPK are required for RAS/ELK signaling, confirming the importance of this ancient pathway and identifying an important role for NLK in modulating muscle development in vivo.

MultiCapsNet: A General Framework for Data Integration and Interpretable Classification

The latest progresses of experimental biology have generated a large number of data with different formats and lengths. Deep learning is an ideal tool to deal with complex datasets, but its inherent “black box” nature needs more interpretability. At the same time, traditional interpretable machine learning methods, such as linear regression or random forest, could only deal with numerical features instead of modular features often encountered in the biological field. Here, we present MultiCapsNet (https://github.com/wanglf19/MultiCapsNet), a new deep learning model built on CapsNet and scCapsNet, which possesses the merits such as easy data integration and high model interpretability. To demonstrate the ability of this model as an interpretable classifier to deal with modular inputs, we test MultiCapsNet on three datasets with different data type and application scenarios. Firstly, on the labeled variant call dataset, MultiCapsNet shows a similar classification performance with neural network model, and provides importance scores for data sources directly without an extra importance determination step required by the neural network model. The importance scores generated by these two models are highly correlated. Secondly, on single cell RNA sequence (scRNA-seq) dataset, MultiCapsNet integrates information about protein-protein interaction (PPI), and protein-DNA interaction (PDI). The classification accuracy of MultiCapsNet is comparable to the neural network and random forest model. Meanwhile, MultiCapsNet reveals how each transcription factor (TF) or PPI cluster node contributes to classification of cell type. Thirdly, we made a comparison between MultiCapsNet and SCENIC. The results show several cell type relevant TFs identified by both methods, further proving the validity and interpretability of the MultiCapsNet.

Age-Related Exosomal and Endogenous Expression Patterns of miR-1, miR-133a, miR-133b, and miR-206 in Skeletal Muscles

Skeletal muscle growth and maintenance depend on two tightly regulated processes, myogenesis and muscle regeneration. Both processes involve a series of crucial regulatory molecules including muscle-specific microRNAs, known as myomiRs. We recently showed that four myomiRs, miR-1, miR-133a, miR-133b, and miR-206, are encapsulated within muscle-derived exosomes and participate in local skeletal muscle communication. Although these four myomiRs have been extensively studied for their function in muscles, no information exists regarding their endogenous and exosomal levels across age. Here we aimed to identify any age-related changes in the endogenous and muscle-derived exosomal myomiR levels during acute skeletal muscle growth. The four endogenous and muscle-derived myomiRs were investigated in five skeletal muscles (extensor digitorum longus, soleus, tibialis anterior, gastrocnemius, and quadriceps) of 2-week–1-year-old wild-type male mice. The expression of miR-1, miR-133a, and miR-133b was found to increase rapidly until adolescence in all skeletal muscles, whereas during adulthood it remained relatively stable. By contrast, endogenous miR-206 levels were observed to decrease with age in all muscles, except for soleus. Differential expression of the four myomiRs is also inversely reflected on the production of two protein targets; serum response factor and connexin 43. Muscle-derived exosomal miR-1, miR-133a, and miR-133b levels were found to increase until the early adolescence, before reaching a plateau phase. Soleus was found to be the only skeletal muscle to release exosomes enriched in miR-206. In this study, we showed for the first time an in-depth longitudinal analysis of the endogenous and exosomal levels of the four myomiRs during skeletal muscle development. We showed that the endogenous expression and extracellular secretion of the four myomiRs are associated to the function and size of skeletal muscles as the mice age. Overall, our findings provide new insights for the myomiRs’ significant role in the first year of life in mice.

Fli1 Promotes Vascular Morphogenesis by Regulating Endothelial Potential of Multipotent Myogenic Progenitors

[Figure: see text].

Disrupted circadian oscillations in type 2 diabetes are linked to altered rhythmic mitochondrial metabolism in skeletal muscle

Circadian rhythms are generated by an autoregulatory feedback loop of transcriptional activators and repressors. Circadian rhythm disruption contributes to type 2 diabetes (T2D) pathogenesis. We elucidated whether altered circadian rhythmicity of clock genes is associated with metabolic dysfunction in T2D. Transcriptional cycling of core-clock genes BMAL1, CLOCK, and PER3 was altered in skeletal muscle from individuals with T2D, and this was coupled with reduced number and amplitude of cycling genes and disturbed circadian oxygen consumption. Inner mitochondria–associated genes were enriched for rhythmic peaks in normal glucose tolerance, but not T2D, and positively correlated with insulin sensitivity. Chromatin immunoprecipitation sequencing identified CLOCK and BMAL1 binding to inner-mitochondrial genes associated with insulin sensitivity, implicating regulation by the core clock. Inner-mitochondria disruption altered core-clock gene expression and free-radical production, phenomena that were restored by resveratrol treatment. We identify bidirectional communication between mitochondrial function and rhythmic gene expression, processes that are disturbed in diabetes.

The Role of Satellite Cells in Skeletal Muscle Regeneration-The Effect of Exercise and Age

Simple Summary

Studies describing the effects of various forms of exercise and age on muscle regeneration were reviewed. Satellite cells are a heterogeneous group of cells that includes stem cells and skeletal muscle progenitor cells. Each skeletal muscle fiber has its own pool of satellite cells that remain inactive until the muscle is damaged. Minor damage within the cell membrane of muscle fibers is patched by fusing intracellular vesicles with the damaged sarcolemma. More severe muscle damage initiates a multistep regeneration process in which satellite cells play an essential role. The condition that initiates the cascade of reactions is the formation of inflammation at the structural discontinuity site, resulting in satellite cell activation. The multitude of reactions and pathways occurring during this process means that many different substances are involved in it and control it. Not all of them are well-understood yet. In parallel, the body’s own population of satellite cells is being rebuilt so that more fibers can be regenerated in the future. Athletes and the elderly are primarily at risk for muscle damage, and pathologies in muscle fiber regeneration cause serious diseases.

Abstract

The population of satellite cells (mSCs) is highly diversified. The cells comprising it differ in their ability to regenerate their own population and differentiate, as well as in the properties they exhibit. The heterogeneity of this group of cells is evidenced by multiple differentiating markers that enable their recognition, classification, labeling, and characterization. One of the main tasks of satellite cells is skeletal muscle regeneration. Myofibers are often damaged during vigorous exercise in people who participate in sports activities. The number of satellite cells and the speed of the regeneration processes that depend on them affect the time structure of an athlete’s training. This process depends on inflammatory cells. The multitude of reactions and pathways that occur during the regeneration process results in the participation and control of many factors that are activated and secreted during muscle fiber damage and at different stages of its regeneration. However, not all of them are well understood yet. This paper presents the current state of knowledge on satellite cell-dependent skeletal muscle regeneration. Studies describing the effects of various forms of exercise and age on this process were reviewed.

MRTF-A regulates myoblast commitment to differentiation by targeting PAX7 during muscle regeneration

Myocardin-related transcription factor-A/serum response factor (MRTF-A/SRF), a well-known transcriptional programme, has been proposed to play crucial roles in skeletal muscle development and function. However, whether MRTF-A participates in muscle regeneration and the molecular mechanisms are not completely understood. Here, we show that MRTF-A levels are highly correlated with myogenic genes using a RNA-seq assay, which reveal that MRTF-A knockdown in C2C12 cells significantly reduces PAX7 expression. Subsequent in vitro and in vivo data show that MRTF-A and PAX7 present identical expression patterns during myoblast differentiation and CTX-induced muscle injury and repair. Remarkably, MRTF-A overexpression promotes myoblast proliferation, while inhibiting cell differentiation and the expression of MyoD and MyoG. MRTF-A loss of function produces the opposite effect. Moreover, mice with lentivirus (MRTF-A) injection possesses more PAX7⁺ satellite cells, but less differentiating MyoD⁺ and MyoG⁺ cells, leading subsequently to diminished muscle regeneration. Our mechanistic results reveal that MRTF-A contributes to PAX7-mediated myoblast self-renewal, proliferation, and differentiation by binding to its distal CArG box. Overall, we propose that MRTF-A functions as a novel PAX7 regulator upon myoblast commitment to differentiation, which could provide pathways for dictating muscle stem cell fate and open new avenues to explore stem cell-based therapy for muscle degenerative diseases.

The Effect of Mechanical Stretch on Myotube Growth Suppression by Colon-26 Tumor-Derived Factors

Preclinical models and in vitro experiments have provided valuable insight into the regulation of cancer-induced muscle wasting. Colon-26 (C26) tumor cells induce cachexia in mice, and conditioned media (CM) from these cells promotes myotube atrophy and catabolic signaling. While mechanical stimuli can prevent some effects of tumor-derived factors on myotubes, the impact of mechanical signaling on tumor-derived factor regulation of myosin heavy chain (MyHC) expression is not well understood. Therefore, we examined the effects of stretch-induced mechanical signaling on C2C12 myotube growth and MyHC expression after C26 CM exposure. C26 CM was administered to myotubes on day 5 of differentiation for 48 h. During the last 4 or 24 h of C26 CM exposure, 5% static uniaxial stretch was administered. C26 CM suppressed myotube growth and MyHC protein and mRNA expression. Stretch for 24 h increased myotube size and prevented the C26 CM suppression of MyHC-Fast protein expression. Stretch did not change suppressed MyHC mRNA expression. Stretch for 24 h reduced Atrogin-1/MAFbx, MuRF-1, and LC3B II/I ratio and increased integrin β1D protein expression and the myogenin-to-MyoD protein ratio. Stretch in the last 4 h of CM increased ERK1/2 phosphorylation but did not alter the CM induction of STAT3 or p38 phosphorylation. These results provide evidence that in myotubes pre-incubated with CM, the induction of mechanical signaling can still provide a growth stimulus and preserve MyHC-Fast protein expression independent of changes in mRNA expression.

Cyclase-associated protein 2 (CAP2) controls MRTF-A localization and SRF activity in mouse embryonic fibroblasts

Recent studies identified cyclase-associated proteins (CAPs) as important regulators of actin dynamics that control assembly and disassembly of actin filaments (F-actin). While these studies significantly advanced our knowledge of their molecular functions, the physiological relevance of CAPs largely remained elusive. Gene targeting in mice implicated CAP2 in heart physiology and skeletal muscle development. Heart defects in CAP2 mutant mice were associated with altered activity of serum response factor (SRF), a transcription factor involved in multiple biological processes including heart function, but also skeletal muscle development. By exploiting mouse embryonic fibroblasts (MEFs) from CAP2 mutant mice, we aimed at deciphering the CAP2-dependent mechanism relevant for SRF activity. Reporter assays and mRNA quantification by qPCR revealed reduced SRF-dependent gene expression in mutant MEFs. Reduced SRF activity in CAP2 mutant MEFs was associated with altered actin turnover, a shift in the actin equilibrium towards monomeric actin (G-actin) as well as and reduced nuclear levels of myocardin-related transcription factor A (MRTF-A), a transcriptional SRF coactivator that is shuttled out of the nucleus and, hence, inhibited upon G-actin binding. Moreover, pharmacological actin manipulation with jasplakinolide restored MRTF-A distribution in mutant MEFs. Our data are in line with a model in which CAP2 controls the MRTF-SRF pathway in an actin-dependent manner. While MRTF-A localization and SRF activity was impaired under basal conditions, serum stimulation induced nuclear MRTF-A translocation and SRF activity in mutant MEFs similar to controls. In summary, our data revealed that in MEFs CAP2 controls basal MRTF-A localization and SRF activity, while it was dispensable for serum-induced nuclear MRTF-A translocation and SRF stimulation.

Transcriptome analysis reveals the long intergenic noncoding RNAs contributed to skeletal muscle differences between Yorkshire and Tibetan pig

The difference between the skeletal muscle growth rates of Western and domestic breeds is remarkable, but the potential regulatory mechanism involved is still unclear. Numerous studies have pointed out that long intergenic noncoding RNA (lincRNA) plays a key role in skeletal muscle development. This study used published Yorkshire (LW) and Tibetan pig (TP) transcriptome data to explore the possible role of lincRNA in the difference in skeletal muscle development between the two breeds. 138 differentially expressed lincRNAs (DELs) were obtained between the two breeds, and their potential target genes (PTGs) were predicted. The results of GO and KEGG analysis revealed that PTGs are involved in multiple biological processes and pathways related to muscle development. The quantitative trait loci (QTLs) of DELs were predicted, and the results showed that most QTLs are related to muscle development. Finally, we constructed a co-expression network between muscle development related PTGs (MDRPTGs) and their corresponding DELs on the basis of their expression levels. The expression of DELs was significantly correlated with the corresponding MDRPTGs. Also, multiple MDRPTGs are involved in the key regulatory pathway of muscle fiber hypertrophy, which is the IGF-1-AKT-mTOR pathway. In summary, multiple lincRNAs that may cause differences in skeletal muscle development between the two breeds were identified, and their possible regulatory roles were explored. The findings of this study may provide a valuable reference for further research on the role of lincRNA in skeletal muscle development.

Epigenetic modifications in muscle regeneration and progression of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a multisystemic disorder that affects 1:5000 boys. The severity of the phenotype varies dependent on the mutation site in the DMD gene and the resultant dystrophin expression profile. In skeletal muscle, dystrophin loss is associated with the disintegration of myofibers and their ineffective regeneration due to defective expansion and differentiation of the muscle stem cell pool. Some of these phenotypic alterations stem from the dystrophin absence-mediated serine-threonine protein kinase 2 (MARK2) misplacement/downregulation in activated muscle stem (satellite) cells and neuronal nitric oxide synthase loss in cells committed to myogenesis. Here, we trace changes in DNA methylation, histone modifications, and expression of regulatory noncoding RNAs during muscle regeneration, from the stage of satellite cells to myofibers. Furthermore, we describe the abrogation of these epigenetic regulatory processes due to changes in signal transduction in DMD and point to therapeutic treatments increasing the regenerative potential of diseased muscles based on this acquired knowledge.

Molecular Mechanisms of Skeletal Muscle Hypertrophy

Skeletal muscle hypertrophy can be induced by hormones and growth factors acting directly as positive regulators of muscle growth or indirectly by neutralizing negative regulators, and by mechanical signals mediating the effect of resistance exercise. Muscle growth during hypertrophy is controlled at the translational level, through the stimulation of protein synthesis, and at the transcriptional level, through the activation of ribosomal RNAs and muscle-specific genes. mTORC1 has a central role in the regulation of both protein synthesis and ribosomal biogenesis. Several transcription factors and co-activators, including MEF2, SRF, PGC-1α4, and YAP promote the growth of the myofibers. Satellite cell proliferation and fusion is involved in some but not all muscle hypertrophy models.

Post-Transcriptional Regulation in Skeletal Muscle Development, Repair, and Disease

Skeletal muscle formation is a complex process that requires tight spatiotemporal control of key myogenic factors. Emerging evidence suggests that RNA processing is crucial for the regulation of these factors, and that multiple post-transcriptional regulatory pathways work dependently and independently of one another to enable precise control of transcripts throughout muscle development and repair. Moreover, disruption of these pathways is implicated in neuromuscular disease, and the recent development of RNA-mediated therapies shows enormous promise in the treatment of these disorders. We discuss the overlapping post-transcriptional regulatory pathways that mediate muscle development, how these pathways are disrupted in neuromuscular disorders, and advances in RNA-mediated therapies that present a novel approach to the treatment of these diseases.

Development and patterning of rib primordia are dependent on associated musculature

The importance of skeletal muscle for rib development and patterning in the mouse embryo has not been resolved, largely because different experimental approaches have yielded disparate results. In this study, we utilize both gene knockouts and muscle cell ablation approaches to re-visit the extent to which rib growth and patterning are dependent on developing musculature. Consistent with previous studies, we show that rib formation is highly dependent on the MYOD family of myogenic regulatory factors (MRFs), and demonstrate that the extent of rib formation is gene-, allele-, and dosage-dependent. In the absence of Myf5 and MyoD, one allele of Mrf4 is sufficient for extensive rib growth, although patterning is abnormal. Under conditions of limiting MRF dosage, MyoD is identified as a positive regulator of rib patterning, presumably due to improved intercostal muscle development. In contrast to previous muscle ablation studies, we show that diphtheria toxin subunit A (DTA)-mediated ablation of muscle progenitors or differentiated muscle, using MyoDiCre or HSA-Cre drivers, respectively, profoundly disrupts rib development. Further, a comparison of three independently derived Rosa26-based DTA knockin alleles demonstrates that the degree of rib perturbations in MyoDiCre/+/DTA embryos is markedly dependent on the DTA allele used, and may in part explain discrepancies with previous findings. The results support the conclusion that the extent and quality of rib formation is largely dependent on the dosage of Myf5 and Mrf4, and that both early myotome-sclerotome interactions, as well as later muscle-rib interactions, are important for proper rib growth and patterning.

MicroRNAs in Skeletal Muscle and Hints on Their Potential Role in Muscle Wasting During Cancer Cachexia

Cancer-associated cachexia is a heterogeneous, multifactorial syndrome characterized by systemic inflammation, unintentional weight loss, and profound alteration in body composition. The main feature of cancer cachexia is represented by the loss of skeletal muscle tissue, which may or may not be accompanied by significant adipose tissue wasting. Such phenotypic alteration occurs as the result of concomitant increased myofibril breakdown and reduced muscle protein synthesis, actively contributing to fatigue, worsening of quality of life, and refractoriness to chemotherapy. According to the classical view, this condition is primarily triggered by interactions between specific tumor-induced pro-inflammatory cytokines and their cognate receptors expressed on the myocyte membrane. This causes a shift in gene expression of muscle cells, eventually leading to a pronounced catabolic condition and cell death. More recent studies, however, have shown the involvement of regulatory non-coding RNAs in the outbreak of cancer cachexia. In particular, the role exerted by microRNAs is being widely addressed, and several mechanistic studies are in progress. In this review, we discuss the most recent findings concerning the role of microRNAs in triggering or exacerbating muscle wasting in cancer cachexia, while mentioning about possible roles played by long non-coding RNAs and ADAR-mediated miRNA modifications.

SRF-MRTF signaling suppresses brown adipocyte development by modulating TGF-β/BMP pathway

The SRF/MRTF and upstream signaling cascade play key roles in actin cytoskeleton organization and myocyte development. To date, how this signaling axis may function in brown adipocyte lineage commitment and maturation has not been delineated. Here we report that MRTF-SRF signaling exerts inhibitory actions on brown adipogenesis, and suppressing this negative regulation promotes brown adipocyte lineage development. During brown adipogenic differentiation, protein expressions of SRF, MRTFA/B and its transcription targets were down-regulated, and MRTFA/B shuttled from nucleus to cytoplasm. Silencing of SRF or MRTF-A/MRTF-B enhanced two distinct stages of brown adipocyte development, mesenchymal stem cell determination to brown adipocytes and terminal differentiation of brown adipogenic progenitors. We further demonstrate that the MRTF-SRF axis exerts transcriptional regulations of the TGF-β and BMP signaling pathway, critical developmental cues for brown adipocyte development. TGF-β signaling activity was significantly attenuated, whereas that of the BMP pathway augmented by inhibition of SRF or MRTF-A/MRTF-B, leading to enhanced brown adipocyte differentiation. Our study demonstrates the MRTF-SRF transcriptional cascade as a negative regulator of brown adipogenesis, through its transcriptional control of the TGF-β/BMP signaling pathways.

Serum response factor-cofactor interactions and their implications in disease

Serum response factor (SRF), a member of the Mcm1, Agamous, Deficiens, and SRF (MADS) box transcription factor, is widely expressed in all cell types and plays a crucial role in the physiological function and development of diseases. SRF regulates its downstream genes by binding to their CArG DNA box by interacting with various cofactors. However, the underlying mechanisms are not fully understood, therefore attracting increasing research attention due to the importance of this topic. This review's objective is to discuss the new progress in the studies of the molecular mechanisms involved in the activation of SRF and its impacts in physiological and pathological conditions. Notably, we summarized the recent studies on the interaction of SRF with its two main types of cofactors belonging to the myocardin families of transcription factors and the members of the ternary complex factors. The knowledge of these mechanisms will create new opportunities for understanding the dynamics of many traits and disease pathogenesis especially, cardiovascular diseases and cancer that could serve as targets for pharmacological control and treatment of these diseases.

The emerging roles of circular RNAs in regulating the fate of stem cells

Circular RNAs(circRNAs) are a large family of RNAs shaping covalently closed ring-like molecules and have become a hotspot with thousands of newly published studies. Stem cells are undifferentiated cells and have great potential in medical treatment due to their self-renewal ability and differentiation capacity. Abundant researches have unveiled that circRNAs have unique expression profile during the differentiation of stem cells and could serve as promising biomarkers of these cells. There are key circRNAs relevant to the differentiation, proliferation, and apoptosis of stem cells with certain mechanisms such as sponging miRNAs, interacting with proteins, and interfering mRNA translation. Moreover, several circRNAs have joined in the interplay between stem cells and lymphocytes. Our review will shed lights on the emerging roles of circRNAs in regulating the fate of diverse stem cells.

SRF-FOXO1 and SRF-NCOA1 Fusion Genes Delineate a Distinctive Subset of Well-differentiated Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) encompasses a heterogenous collection of tumors in which new groups have recently been identified that improved the World Health Organization (WHO) classification. While performing RNA-sequencing in our routine practice, we identified 3 cases of well-differentiated RMS harboring new fusion genes. We also analyzed these tumors through array-comparative genomic hybridization. Clinically, these tumors were deep paraspinal tumors, occurring in neo-nat and young children. The patients underwent resection and adjuvant therapy. At the time of last follow-up (ranging from 12 to 108 mo), they were alive without disease. Histologically, these tumors consisted of well-differentiated rhabdomyoblastic proliferations with nuclear atypia, infiltrative borders, and a specific growth pattern. These tumors harbored new fusion genes involving SRF and either FOXO1 or NCOA1. We compared the expression profiles of these 3 tumors to the expression data of a series of 33 skeletal muscle tumors including embryonal RMSs, alveolar rhandomyosarcomas, RMSs with VGLL2 fusions, RMSs with the myoD1 mutation, EWSR1/FUS-TFCP2 epithelioid and spindle cell RMSs of the bone, and rhabdomyomas with PTCH1 loss. According to clustering analyses, the 3 SRF-fused tumors formed a distinct group with a specific expression profile different from that of the other types of skeletal muscle tumors. Array-comparative genomic hybridization showed a recurrent gain of chromosome 11. These 3 tumors define a new group of RMS associated with a fusion of the SRF gene. FOXO1 rearrangements, usually used to confirm the diagnosis of alveolar RMS and identify poor-outcome RMSs, were identified in a nonalveolar RMS for the first time.

Tissue-specific disruption of Kbtbd2 uncovers adipocyte-intrinsic and -extrinsic features of the teeny lipodystrophy syndrome

To examine the function of KBTBD2 in different cell types, we knocked the gene out in adipocytes, liver, and muscle cells. None of these conditional knockouts could reproduce the growth retardation observed in mice with a global knockout. An adipose tissue-specific knockout was similar to a global knockout in most respects; however, hyperglycemia was less severe and hyperinsulinemia was more sustained. An adipose tissue-specific knockout caused insulin resistance in other tissues, demonstrating an adipose-extrinsic effect of the mutation.

Loss of KBTBD2 in all tissues causes the teeny phenotype, characterized by insulin resistance with late failure of insulin production, severe hyperglycemia/diabetes, lipodystrophy, hepatosteatosis, and growth retardation. KBTBD2 maintains insulin sensitivity in adipocytes by restricting the abundance of p85α. However, the possible physiological contribution or contributions of KBTBD2 have not yet been examined in other tissues. Here we show that mice with an adipocyte-specific knockout of Kbtbd2 accumulate p85α in white and brown adipose tissues, causing insulin resistance, moderate rather than severe hyperglycemia, sustained hyperinsulinemia without late failure of insulin production, and lipodystrophy leading to ectopic lipid accumulation in the liver. Adipocyte-extrinsic insulin resistance was observed in liver and muscle. None of these abnormalities were observed in liver- or muscle-specific Kbtbd2 knockout mice. Mice with Kbtbd2 knockout in adipocytes, liver, and muscle all showed normal growth, suggesting that KBTBD2 may be necessary to ensure IGF1 signaling in other tissues, notably bone. While much of the teeny phenotype results from loss of KBTBD2 in adipocytes, some features are adipocyte-extrinsic.

Myofibers deficient in connexins 43 and 45 expression protect mice from skeletal muscle and systemic dysfunction promoted by a dysferlin mutation

Dysferlinopathy is a genetic human disease caused by mutations in the gene that encodes the dysferlin protein (DYSF). Dysferlin is believed to play a relevant role in cell membrane repair. However, in dysferlin-deficient (blAJ) mice (a model of dysferlinopathies) the recovery of the membrane resealing function by means of the expression of a mini-dysferlin does not arrest progressive muscular damage, suggesting the participation of other unknown pathogenic mechanisms. Here, we show that proteins called connexins 39, 43 and 45 (Cx39, Cx43 and Cx45, respectively) are expressed by blAJ myofibers and form functional hemichannels (Cx HCs) in the sarcolemma. At rest, Cx HCs increased the sarcolemma permeability to small molecules and the intracellular Ca²⁺ signal. In addition, skeletal muscles of blAJ mice showed lipid accumulation and lack of dysferlin immunoreactivity. As sign of extensive damage and atrophy, muscles of blAJ mice presented elevated numbers of myofibers with internal nuclei, increased number of myofibers with reduced cross-sectional area and elevated creatine kinase activity in serum. In agreement with the extense muscle damage, mice also showed significantly low motor performance. We generated blAJ mice with myofibers deficient in Cx43 and Cx45 expression and found that all above muscle and systemic alterations were absent, indicating that these two Cxs play a critical role in a novel pathogenic mechanism of dysfernolophaties, which is discussed herein. Therefore, Cx HCs could constitute an attractive target for pharmacologic treatment of dyferlinopathies.

Interference with SRF expression in skeletal muscles reduces peripheral nerve regeneration in mice

Traumatic injury of peripheral nerves typically also damages nerve surrounding tissue including muscles. Hence, molecular and cellular interactions of neighboring damaged tissues might be decisive for successful axonal regeneration of injured nerves. So far, the contribution of muscles and muscle-derived molecules to peripheral nerve regeneration has only poorly been studied. Herein, we conditionally ablated SRF (serum response factor), an important myofiber transcription factor, in skeletal muscles of mice. Subsequently, the impact of this myofiber-restricted SRF deletion on peripheral nerve regeneration, i.e. facial nerve injury was analyzed. Quantification of facial nerve regeneration by retrograde tracer transport, inspection of neuromuscular junctions (NMJs) and recovery of whisker movement revealed reduced axonal regeneration upon muscle specific Srf deletion. In contrast, responses in brainstem facial motor neuron cell bodies such as regeneration-associated gene (RAG) induction of Atf3, synaptic stripping and neuroinflammation were not overly affected by SRF deficiency. Mechanistically, SRF in myofibers appears to stimulate nerve regeneration through regulation of muscular satellite cell (SC) proliferation. In summary, our data suggest a role of muscle cells and SRF expression within muscles for regeneration of injured peripheral nerves.

MEG3 Promotes Differentiation of Porcine Satellite Cells by Sponging miR-423-5p to Relieve Inhibiting Effect on SRF

Although thousands of long noncoding RNAs (lncRNAs) have been identified in porcine growth and development, the regulation mechanisms of functional lncRNAs have not been well explored. In this study, using 5′- and 3′-rapid amplification of cDNA ends (RACE) assays, we obtained two different variants of lncRNA maternally expressed gene 3 (MEG3), namely, MEG3 v1 and MEG3 v2, that were both highly expressed in porcine skeletal muscle and in the early stage of the differentiation of porcine satellite cells. Moreover, we identified the core transcript MEG3 v2. Functional analyses showed that MEG3 overexpression could effectively arrest myoblasts in the G1 phase, inhibit DNA replication, and promote myoblast differentiation, whereas MEG3 knockdown resulted in the opposite effects. Interestingly, the expression of serum response factor (SRF), a crucial transcription factor for myogenesis process, remarkably increased and decreased in mRNA and protein levels with the respective overexpression and knockdown of MEG3. Dual luciferase reporter assay showed that MEG3 could attenuate the decrease of luciferase activity of SRF induced by miR-423-5p in a dose-dependent manner. MEG3 overexpression could relieve the inhibitory effect on SRF and myoblast differentiation induced by miR-423-5p. In addition, results of RNA immunoprecipitation analysis suggested that MEG3 could act as a ceRNA for miR-423-5p. Our findings initially established a novel connection among MEG3, miR-423-5p, and SRF in porcine satellite cell differentiation. This novel role of MEG3 may shed new light on understanding of molecular regulation of lncRNA in porcine myogenesis.

Mechanisms regulating myoblast fusion: A multilevel interplay

Myoblast fusion into myotubes is one of the crucial steps of skeletal muscle development (myogenesis). The fusion is preceded by specification of a myogenic lineage (mesodermal progenitors) differentiating into myoblasts and is followed by myofiber-type specification and neuromuscular junction formation. Similarly to other processes of myogenesis, the fusion requires a very precise spatial and temporal regulation occuring both during embryonic development as well as regeneration and repair of the muscle. A plethora of genes and their products is involved in regulation of myoblast fusion and a precise multilevel interplay between them is crucial for myogenic cells to fuse. In this review, we describe both cellular events taking place during myoblast fusion (migration, adhesion, elongation, cell-cell recognition, alignment, and fusion of myoblast membranes enabling formation of myotubes) as well as recent findings on mechanisms regulating this process. Also, we present muscle disorders in humans that have been associated with defects in genes involved in regulation of myoblast fusion.

Myopalladin promotes muscle growth through modulation of the serum response factor pathway

BACKGROUND

Myopalladin (MYPN) is a striated muscle-specific, immunoglobulin-containing protein located in the Z-line and I-band of the sarcomere as well as the nucleus. Heterozygous MYPN gene mutations are associated with hypertrophic, dilated, and restrictive cardiomyopathy, and homozygous loss-of-function truncating mutations have recently been identified in patients with cap myopathy, nemaline myopathy, and congenital myopathy with hanging big toe.

METHODS

Constitutive MYPN knockout (MKO) mice were generated, and the role of MYPN in skeletal muscle was studied through molecular, cellular, biochemical, structural, biomechanical, and physiological studies in vivo and in vitro.

RESULTS

MKO mice were 13% smaller compared with wild-type controls and exhibited a 48% reduction in myofibre cross-sectional area (CSA) and significantly increased fibre number. Similarly, reduced myotube width was observed in MKO primary myoblast cultures. Biomechanical studies showed reduced isometric force and power output in MKO mice as a result of the reduced CSA, whereas the force developed by each myosin molecular motor was unaffected. While the performance by treadmill running was similar in MKO and wild-type mice, MKO mice showed progressively decreased exercise capability, Z-line damage, and signs of muscle regeneration following consecutive days of downhill running. Additionally, MKO muscle exhibited progressive Z-line widening starting from 8 months of age. RNA-sequencing analysis revealed down-regulation of serum response factor (SRF)-target genes in muscles from postnatal MKO mice, important for muscle growth and differentiation. The SRF pathway is regulated by actin dynamics as binding of globular actin to the SRF-cofactor myocardin-related transcription factor A (MRTF-A) prevents its translocation to the nucleus where it binds and activates SRF. MYPN was found to bind and bundle filamentous actin as well as interact with MRTF-A. In particular, while MYPN reduced actin polymerization, it strongly inhibited actin depolymerization and consequently increased MRTF-A-mediated activation of SRF signalling in myogenic cells. Reduced myotube width in MKO primary myoblast cultures was rescued by transduction with constitutive active SRF, demonstrating that MYPN promotes skeletal muscle growth through activation of the SRF pathway.

CONCLUSIONS

Myopalladin plays a critical role in the control of skeletal muscle growth through its effect on actin dynamics and consequently the SRF pathway. In addition, MYPN is important for the maintenance of Z-line integrity during exercise and aging. These results suggest that muscle weakness in patients with biallelic MYPN mutations may be associated with reduced myofibre CSA and SRF signalling and that the disease phenotype may be aggravated by exercise.

Enzymatically crosslinked gelatin-laminin hydrogels for applications in neuromuscular tissue engineering

We report a water-soluble and non-toxic method to incorporate additional extracellular matrix proteins into gelatin hydrogels, while obviating the use of chemical crosslinkers such as glutaraldehyde. Gelatin hydrogels were fabricated using a range of gelatin concentrations (4%-10%) that corresponded to elastic moduli of approximately 1 kPa-25 kPa, respectively, a substrate stiffness relevant for multiple cell types. Microbial transglutaminase was then used to enzymatically crosslink a layer of laminin on top of gelatin hydrogels, resulting in 2-component gelatin-laminin hydrogels. Human induced pluripotent stem cell derived spinal spheroids readily adhered and rapidly extended axons on GEL-LN hydrogels. Axons displayed a more mature morphology and superior electrophysiological properties on GEL-LN hydrogels compared to the controls. Schwann cells on GEL-LN hydrogels adhered and proliferated normally, displayed a healthy morphology, and maintained the expression of Schwann cell specific markers. Lastly, skeletal muscle cells on GEL-LN hydrogels achieved long-term culture for up to 28 days without delamination, while expressing higher levels of terminal genes including myosin heavy chain, MyoD, MuSK, and M-cadherin suggesting enhanced maturation potential and myotube formation compared to the controls. Future studies will employ the superior culture outcomes of this hybrid substrate for engineering functional neuromuscular junctions and related organ on a chip applications.

Regulation of SRF protein stability by an autophagy-dependent pathway

Serum response factor (SRF), a key transcription factor, plays an important role in regulating cell functions such as proliferation and differentiation. Most proteins are unstable, and protein stability is regulated through the ubiquitin-proteasome system (UPS) or the autophagy lysosome pathway (ALP). Whether SRF is degraded and what mechanisms control SRF protein stability remain unexplored. Western blot analyses of cells treated with cycloheximide (CHX), a protein synthesis inhibitor, showed that SRF was degraded in a time-dependent manner. Moreover, we observed that SRF undergoes autophagy-dependent destruction, which is accelerated by serum deprivation. Through bioinformatics screening, we found that SRF contains the GSK3β phosphorylation motif (T/SPPXS): SPDSPPRSDPT, which is conserved from zebrafish to humans. Serum deprivation stimulated GSK3β activation that then potentiates SRF degradation through the autophagy lysosome pathway. Since SRF is important for numerous cellular activities, our results suggest that the autophagy-dependent SRF degradation pathway may provide a new avenue to modulate SRF-mediated cell functions.

Loss of mitochondrial energetics is associated with poor recovery of muscle function but not mass following disuse atrophy

Skeletal muscle atrophy is a clinically important outcome of disuse because of injury, immobilization, or bed rest. Disuse atrophy is accompanied by mitochondrial dysfunction, which likely contributes to activation of the muscle atrophy program. However, the linkage of muscle mass and mitochondrial energetics during disuse atrophy and its recovery is incompletely understood. Transcriptomic analysis of muscle biopsies from healthy older adults subject to complete bed rest revealed marked inhibition of mitochondrial energy metabolic pathways. To determine the temporal sequence of muscle atrophy and changes in intramyocellular lipid and mitochondrial energetics, we conducted a time course of hind limb unloading-induced atrophy in adult mice. Mitochondrial respiration and calcium retention capacity were diminished, whereas H2O2 emission was increased within 3 days of unloading before significant muscle atrophy. These changes were associated with a decrease in total cardiolipin and profound changes in remodeled cardiolipin species. Hind limb unloading performed in muscle-specific peroxisome proliferator-activated receptor-γ coactivator-1α/β knockout mice, a model of mitochondrial dysfunction, did not affect muscle atrophy but impacted muscle function. These data suggest early mitochondrial remodeling affects muscle function but not mass during disuse atrophy. Early alterations in mitochondrial energetics and lipid remodeling may represent novel targets to prevent muscle functional impairment caused by disuse and to enhance recovery from periods of muscle atrophy.