Six1 and Six4 gene expression is necessary to activate the fast-type muscle gene program in the mouse primary myotome

Dynamic Changes in the Global Transcriptome of Postnatal Skeletal Muscle in Different Sheep

Sheep growth performance, mainly skeletal muscle growth, provides direct economic benefits to the animal husbandry industry. However, the underlying genetic mechanisms of different breeds remain unclear. We found that the cross-sectional area (CSA) of skeletal muscle in Dorper (D) and binary cross-breeding (HD) was higher than that in Hu sheep (H) from 3 months to 12 months after birth. The transcriptomic analysis of 42 quadriceps femoris samples showed that a total of 5053 differential expression genes (DEGs) were identified. The differences in the global gene expression patterns, the dynamic transcriptome of skeletal muscle development, and the transcriptome of the transformation of fast and slow muscles were explored using weighted correlation network analysis (WGCNA) and allele-specific expression analysis. Moreover, the gene expression patterns of HD were more similar to D rather than H from 3 months to 12 months, which might be the reason for the difference in muscle growth in the three breeds. Additionally, several genes (GNB2L1, RPL15, DVL1, FBXO31, etc.) were identified as candidates related to skeletal muscle growth. These results should serve as an important resource revealing the molecular basis of muscle growth and development in sheep.

Developmental, physiologic and phylogenetic perspectives on the expression and regulation of myosin heavy chains in mammalian skeletal muscles

The kinetics of myosin controls the speed and power of muscle contraction. Mammalian skeletal muscles express twelve kinetically different myosin heavy chain (MyHC) genes which provides a wide range of muscle speeds to meet different functional demands. Myogenic progenitors from diverse craniofacial and somitic mesoderm specify muscle allotypes with different repertoires for MyHC expression. This review provides a brief synopsis on the historical and current views on how cell lineage, neural impulse patterns, and thyroid hormone influence MyHC gene expression in muscles of the limb allotype during development and in adult life and the molecular mechanisms thereof. During somitic myogenesis, embryonic and foetal myoblast lineages form slow and fast primary and secondary myotube ontotypes which respond differently to postnatal neural and thyroidal influences to generate fully differentiated fibre phenotypes. Fibres of a given phenotype may arise from myotubes of different ontotypes which retain their capacity to respond differently to neural and thyroidal influences during postnatal life. This gives muscles physiological plasticity to adapt to fluctuations in thyroid hormone levels and patterns of use. The kinetics of MyHC isoforms vary inversely with animal body mass. Fast 2b fibres are specifically absent in muscles involved in elastic energy saving in hopping marsupials and generally absent in large eutherian mammals. Changes in MyHC expression are viewed in the context of the physiology of the whole animal. The roles of myoblast lineage and thyroid hormone in regulating MyHC gene expression are phylogenetically the most ancient while that of neural impulse patterns the most recent.

Overlapping functions of SIX homeoproteins during embryonic myogenesis

Four SIX homeoproteins display a combinatorial expression throughout embryonic developmental myogenesis and they modulate the expression of the myogenic regulatory factors. Here, we provide a deep characterization of their role in distinct mouse developmental territories. We showed, at the hypaxial level, that the Six1:Six4 double knockout (dKO) somitic precursor cells adopt a smooth muscle fate and lose their myogenic identity. At the epaxial level, we demonstrated by the analysis of Six quadruple KO (qKO) embryos, that SIX are required for fetal myogenesis, and for the maintenance of PAX7+ progenitor cells, which differentiated prematurely and are lost by the end of fetal development in qKO embryos. Finally, we showed that Six1 and Six2 are required to establish craniofacial myogenesis by controlling the expression of Myf5. We have thus described an unknown role for SIX proteins in the control of myogenesis at different embryonic levels and refined their involvement in the genetic cascades operating at the head level and in the genesis of myogenic stem cells.

Acoel single-cell atlas reveals expression dynamics and heterogeneity of adult pluripotent stem cells

Adult pluripotent stem cell (aPSC) populations underlie whole-body regeneration in many distantly-related animal lineages, but how the underlying cellular and molecular mechanisms compare across species is unknown. Here, we apply single-cell RNA sequencing to profile transcriptional cell states of the acoel worm Hofstenia miamia during postembryonic development and regeneration. We identify cell types shared across stages and their associated gene expression dynamics during regeneration. Functional studies confirm that the aPSCs, also known as neoblasts, are the source of differentiated cells and reveal transcription factors needed for differentiation. Subclustering of neoblasts recovers transcriptionally distinct subpopulations, the majority of which are likely specialized to differentiated lineages. One neoblast subset, showing enriched expression of the histone variant H3.3, appears to lack specialization. Altogether, the cell states identified in this study facilitate comparisons to other species and enable future studies of stem cell fate potentials.

Large Maf transcription factor family is a major regulator of fast type IIb myofiber determination

Myofibers are broadly characterized as fatigue-resistant slow-twitch (type I) fibers and rapidly fatiguing fast-twitch (type IIa/IIx/IIb) fibers. However, the molecular regulation of myofiber type is not entirely understood; particularly, information on regulators of fast-twitch muscle is scarce. Here, we demonstrate that the large Maf transcription factor family dictates fast type IIb myofiber specification in mice. Remarkably, the ablation of three large Mafs leads to the drastic loss of type IIb myofibers, resulting in enhanced endurance capacity and the reduction of muscle force. Conversely, the overexpression of each large Maf in the type I soleus muscle induces type IIb myofibers. Mechanistically, a large Maf directly binds to the Maf recognition element on the promoter of myosin heavy chain 4, which encodes the type IIb myosin heavy chain, driving its expression. This work identifies the large Maf transcription factor family as a major regulator for fast type IIb muscle determination.

The role of Eya1 and Eya2 in the taste system of mice from embryonic stage to adulthood

Members of the Eya family, which are a class of transcription factors with phosphatase activity, are widely expressed in cranial sensory organs during development. However, it is unclear whether these genes are expressed in the taste system during development and whether they play any role in specifying taste cell fate. In this study, we report that Eya1 is not expressed during embryonic tongue development but that Eya1-expressing progenitors in somites or pharyngeal endoderm give rise to tongue musculature or taste organs, respectively. In the Eya1-deficient tongues, these progenitors do not proliferate properly, resulting in a smaller tongue at birth, impaired growth of taste papillae, and disrupted expression of Six1 in the papillary epithelium. On the other hand, Eya2 is specifically expressed in endoderm-derived circumvallate and foliate papillae located on the posterior tongue during development. In adult tongues, Eya1 is predominantly expressed in IP3R3-positive taste cells in the taste buds of the circumvallate and foliate papillae, while Eya2 is persistently expressed in these papillae at higher levels in some epithelial progenitors and at lower levels in some taste cells. We found that conditional knockout of Eya1 in the third week or Eya2 knockout reduced Pou2f3+, Six1+ and IP3R3+ taste cells. Our data define for the first time the expression patterns of Eya1 and Eya2 during the development and maintenance of the mouse taste system and suggest that Eya1 and Eya2 may act together to promote lineage commitment of taste cell subtypes.

Transcriptome-based analysis of early post-mortem formation of pale, soft, and exudative (PSE) pork

Pale, soft, and exudative (PSE) meat can cause consumer dissatisfaction and economic losses. This study determined meat quality, glycolytic enzyme activity, and differential gene expression in the longissimus lumborum (LL) and semimembranosus (SM) of normal and PSE pork carcasses. The SM did not result in PSE meat. Hexokinase, lactate dehydrogenase, and pyruvate kinase activities were lower in the SM of PSE carcasses than in the normal carcasses. Functional enrichment analysis revealed that immune, inflammatory, and muscle fibre genes were significantly enriched in PSE pork. More specifically, PPP1R3G and MSS51 may be key genes regulating pork quality in the SM. Meanwhile, the differential expression of PLVAB, ADIPOQ, LEP, MYH4, MYH7, MYL3, MYL6B, FOS, ATF3, and HSPA6 may induce PSE formation in the LL. These results may provide insights into PSE pork formation mechanisms and reveal candidate genes for improving meat quality after validation.

CircCSDE1 Regulates Proliferation and Differentiation of C2C12 Myoblasts by Sponging miR-21-3p

The growth and development of skeletal muscle is regulated by many factors, and recent studies have shown that circular RNAs (circRNAs) can participate in this process. The model of porcine skeletal muscle injury was constructed to search for circRNAs that can regulate the growth and development of skeletal muscle in pigs. Using whole-transcriptome sequencing and bioinformatics analysis, a novel circRNA (circCSDE1) was screened out, which is highly expressed in skeletal muscle. Functional studies in C2C12 cells demonstrated that circCSDE1 could promote proliferation and inhibit myoblast differentiation, while opposing changes were observed by circCSDE1 knockdown. A dual-luciferase reporter assay revealed that circCSDE1 directly targeted miR-21-3p to regulate the expression of the downstream target gene (Cyclin-dependent kinase 16, CDK16). Moreover, miR-21-3p could inhibit proliferation and promote myoblast differentiation in C2C12 cells, opposite with the effects of circCSDE1. Additionally, the rescue experiments offered further evidence that circCSDE1 and its target, miR-21-3p, work together to regulate myoblast proliferation and differentiation. This study provides a theoretical basis for further understanding the regulatory mechanisms of circRNAs.

Driving an Oxidative Phenotype Protects Myh4 Null Mice From Myofiber Loss During Postnatal Growth

Postnatal muscle growth is accompanied by increases in fast fiber type compositions and hypertrophy, raising the possibility that a slow to fast transition may be partially requisite for increases in muscle mass. To test this hypothesis, we ablated the Myh4 gene, and thus myosin heavy chain IIB protein and corresponding fibers in mice, and examined its consequences on postnatal muscle growth. Wild-type and Myh4^–/– mice had the same number of muscle fibers at 2 weeks postnatal. However, the gastrocnemius muscle lost up to 50% of its fibers between 2 and 4 weeks of age, though stabilizing thereafter. To compensate for the lack of functional IIB fibers, type I, IIA, and IIX(D) fibers increased in prevalence and size. To address whether slowing the slow-to-fast fiber transition process would rescue fiber loss in Myh4^–/– mice, we stimulated the oxidative program in muscle of Myh4^–/– mice either by overexpression of PGC-1α, a well-established model for fast-to-slow fiber transition, or by feeding mice AICAR, a potent AMP kinase agonist. Forcing an oxidative metabolism in muscle only partially protected the gastrocnemius muscle from loss of fibers in Myh4^–/– mice. To explore whether traditional means of stimulating muscle hypertrophy could overcome the muscling deficits in postnatal Myh4^–/– mice, myostatin null mice were bred with Myh4^–/– mice, or Myh4^–/– mice were fed the growth promotant clenbuterol. Interestingly, both genetic and pharmacological stimulations had little impact on mice lacking a functional Myh4 gene suggesting that the existing muscle fibers have maximized its capacity to enlarge to compensate for the lack of its neighboring IIB fibers. Curiously, however, cell signaling events responsible for IIB fiber formation remained intact in the tissue. These findings further show disrupting the slow-to-fast transition of muscle fibers compromises muscle growth postnatally and suggest that type IIB myosin heavy chain expression and its corresponding fiber type may be necessary for fiber maintenance, transition and hypertrophy in mice. The fact that forcing muscle metabolism toward a more oxidative phenotype can partially compensates for the lack of an intact Myh4 gene provides new avenues for attenuating the loss of fast-twitch fibers in aged or diseased muscles.

SIX1 reprograms myogenic transcription factors to maintain the rhabdomyosarcoma undifferentiated state

Rhabdomyosarcoma (RMS) is a pediatric muscle sarcoma characterized by expression of the myogenic lineage transcription factors (TFs) MYOD1 and MYOG. Despite high expression of these TFs, RMS cells fail to terminally differentiate, suggesting the presence of factors that alter their functions. Here, we demonstrate that the developmental TF SIX1 is highly expressed in RMS and critical for maintaining a muscle progenitor-like state. SIX1 loss induces differentiation of RMS cells into myotube-like cells and impedes tumor growth in vivo. We show that SIX1 maintains the RMS undifferentiated state by controlling enhancer activity and MYOD1 occupancy at loci more permissive to tumor growth over muscle differentiation. Finally, we demonstrate that a gene signature derived from SIX1 loss correlates with differentiation status and predicts RMS progression in human disease. Our findings demonstrate a master regulatory role of SIX1 in repression of RMS differentiation via genome-wide alterations in MYOD1 and MYOG-mediated transcription.

Six1 promotes skeletal muscle thyroid hormone response through regulation of the MCT10 transporter

BACKGROUND

The Six1 transcription factor is implicated in controlling the development of several tissue types, notably skeletal muscle. Six1 also contributes to muscle metabolism and its activity is associated with the fast-twitch, glycolytic phenotype. Six1 regulates the expression of certain genes of the fast muscle program by directly stimulating their transcription or indirectly acting through a long non-coding RNA. We hypothesized that additional mechanisms of action of Six1 might be at play.

METHODS

A combined analysis of gene expression profiling and genome-wide location analysis data was performed. Results were validated using in vivo RNA interference loss-of-function assays followed by measurement of gene expression by RT-PCR and transcriptional reporter assays.

RESULTS

The Slc16a10 gene, encoding the thyroid hormone transmembrane transporter MCT10, was identified as a gene with a transcriptional enhancer directly bound by Six1 and requiring Six1 activity for full expression in adult mouse tibialis anterior, a predominantly fast-twitch muscle. Of the various thyroid hormone transporters, MCT10 mRNA was found to be the most abundant in skeletal muscle, and to have a stronger expression in fast-twitch compared to slow-twitch muscle groups. Loss-of-function of MCT10 in the tibialis anterior recapitulated the effect of Six1 on the expression of fast-twitch muscle genes and led to lower activity of a thyroid hormone receptor-dependent reporter gene.

CONCLUSIONS

These results shed light on the molecular mechanisms controlling the tissue expression profile of MCT10 and identify modulation of the thyroid hormone signaling pathway as an additional mechanism by which Six1 influences skeletal muscle metabolism.

Troponin Variants as Markers of Skeletal Muscle Health and Diseases

Ca2 +-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Cell autonomous requirement of neurofibromin (Nf1) for postnatal muscle hypertrophic growth and metabolic homeostasis

BACKGROUND

Neurofibromatosis type 1 (NF1) is a multi-organ disease caused by mutations in neurofibromin 1 (NF1). Amongst other features, NF1 patients frequently show reduced muscle mass and strength, impairing patients' mobility and increasing the risk of fall. The role of Nf1 in muscle and the cause for the NF1-associated myopathy are mostly unknown.

METHODS

To dissect the function of Nf1 in muscle, we created muscle-specific knockout mouse models for NF1, inactivating Nf1 in the prenatal myogenic lineage either under the Lbx1 promoter or under the Myf5 promoter. Mice were analysed during prenatal and postnatal myogenesis and muscle growth.

RESULTS

Nf1^Lbx1 and Nf1^Myf5 animals showed only mild defects in prenatal myogenesis. Nf1^Lbx1 animals were perinatally lethal, while Nf1^Myf5 animals survived only up to approximately 25 weeks. A comprehensive phenotypic characterization of Nf1^Myf5 animals showed decreased postnatal growth, reduced muscle size, and fast fibre atrophy. Proteome and transcriptome analyses of muscle tissue indicated decreased protein synthesis and increased proteasomal degradation, and decreased glycolytic and increased oxidative activity in muscle tissue. High-resolution respirometry confirmed enhanced oxidative metabolism in Nf1^Myf5 muscles, which was concomitant to a fibre type shift from type 2B to type 2A and type 1. Moreover, Nf1^Myf5 muscles showed hallmarks of decreased activation of mTORC1 and increased expression of atrogenes. Remarkably, loss of Nf1 promoted a robust activation of AMPK with a gene expression profile indicative of increased fatty acid catabolism. Additionally, we observed a strong induction of genes encoding catabolic cytokines in muscle Nf1^Myf5 animals, in line with a drastic reduction of white, but not brown adipose tissue.

CONCLUSIONS

Our results demonstrate a cell autonomous role for Nf1 in myogenic cells during postnatal muscle growth required for metabolic and proteostatic homeostasis. Furthermore, Nf1 deficiency in muscle drives cross-tissue communication and mobilization of lipid reserves.

Myogenesis control by SIX transcriptional complexes

SIX homeoproteins were first described in Drosophila, where they participate in the Pax-Six-Eya-Dach (PSED) network with eyeless, eyes absent and dachsund to drive synergistically eye development through genetic and biochemical interactions. The role of the PSED network and SIX proteins in muscle formation in vertebrates was subsequently identified. Evolutionary conserved interactions with EYA and DACH proteins underlie the activity of SIX transcriptional complexes (STC) both during embryogenesis and in adult myofibers. Six genes are expressed throughout muscle development, in embryonic and adult proliferating myogenic stem cells and in fetal and adult post-mitotic myofibers, where SIX proteins regulate the expression of various categories of genes. In vivo, SIX proteins control many steps of muscle development, acting through feedforward mechanisms: in the embryo for myogenic fate acquisition through the direct control of Myogenic Regulatory Factors; in adult myofibers for their contraction/relaxation and fatigability properties through the control of genes involved in metabolism, sarcomeric organization and calcium homeostasis. Furthermore, during development and in the adult, SIX homeoproteins participate in the genesis and the maintenance of myofibers diversity.

Muscle precursor cell movements in zebrafish are dynamic and require Six family genes

Muscle precursors need to be correctly positioned during embryonic development for proper body movement. In zebrafish, a subset of hypaxial muscle precursors from the anterior somites undergo long-range migration, moving away from the trunk in three streams to form muscles in distal locations such as the fin. We mapped long-distance muscle precursor migrations with unprecedented resolution using live imaging. We identified conserved genes necessary for normal precursor motility (six1a, six1b, six4a, six4b and met). These genes are required for movement away from somites and later to partition two muscles within the fin bud. During normal development, the middle muscle precursor stream initially populates the fin bud, then the remainder of this stream contributes to the posterior hypaxial muscle. When we block fin bud development by impairing retinoic acid synthesis or Fgfr function, the entire stream contributes to the posterior hypaxial muscle indicating that muscle precursors are not committed to the fin during migration. Our findings demonstrate a conserved muscle precursor motility pathway, identify dynamic cell movements that generate posterior hypaxial and fin muscles, and demonstrate flexibility in muscle precursor fates.

The application of gene marker-assisted selection and proteomics for the best meat quality criteria and body measurements in Qinchuan cattle breed

In the past few decades, enhancement of animal productivity has been gaining increasing attention among decisions-makers, politicians, mangers, and breeders, because of the increasing of world population and shortage of natural resources. The selection of high productivity animals is the main goal, through the application of genetic improvement programs. The use of molecular genetics has conferred significant breeding advantages over conventional breeding techniques. In this regard, many economic characteristics are controlled by a small number of multiple gene loci, each of which is responsible for trait diversity and hence they are referred to as quantitative trait loci (QTL). Single-nucleotide polymorphisms (SNPs), which have recently been discovered through DNA sequencing, are considered one of the most useful types of genetic marker. SNPs are found where different nucleotides occur at the same position in the DNA sequence. They are found in both coding and noncoding regions of the genome and are present at one SNP in every 1000 b. Strategies for the identification and application of markers are based on reference to examples of loci that can control various traits. Furthermore, markers for growth, body measurements, and meat quality traits are preferred, because they can be used to predict the performance of animals, via blood samples, in the first few days of animal life. Marker-assisted selection using SNPs, such asSIRT1, SIRT2, LPL, CRTC2, SIX4, UCPs, and ZBTB38as selection criteria of body measurements and meat traits in beef cattle, will be beneficial in selection and breeding programs. The proteomic is a novel marker and a new approache of biotechnology which increases the understanding of the biological processes, besides being a remarkable biomarker that interrelated to growth and meat quality traits. Proteomics is a vigorous tool as usage for deduces molecular processes between quality traits and muscle proteins, which are helpful in analyzing the mechanisms of biochemistry that influence quality. So they could be potential biomarker for some meat quality traits. Among them, Actin, Myosin, Heat shock proteins are used a novel approaches in the field of biotechnology to understand the proteomics changes. This review article highlights the novel findings on the potential use of MAS and proteomics as biomarker for the selection for meat quality and carcass traits in Qinchuan cattle breed.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Characterization of the promoter region of the bovine SIX1 gene: Roles of MyoD, PAX7, CREB and MyoG

The SIX1 gene belongs to the family of six homeodomain transcription factors (TFs), that regulates the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and mediate skeletal muscle growth and regeneration. Previous studies have demonstrated that SIX1 is positively correlated with body measurement traits (BMTs). However, the transcriptional regulation of SIX1 remains unclear. In the present study, we determined that bovine SIX1 was highly expressed in the longissimus thoracis. To elucidate the molecular mechanisms involved in bovine SIX1 regulation, 2-kb of the 5' regulatory region were obtained. Sequence analysis identified neither a consensus TATA box nor a CCAAT box in the 5' flanking region of bovine SIX1. However, a CpG island was predicted in the region -235 to +658 relative to the transcriptional start site (TSS). An electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay in combination with serial deletion constructs of the 5' flanking region, site-directed mutation and siRNA interference demonstrated that MyoD, PAX7 and CREB binding occur in region -689/-40 and play important roles in bovine SIX1 transcription. In addition, MyoG drives SIX1 transcription indirectly via the MEF3 motif. Taken together these interactions suggest a key functional role for SIX1 in mediating skeletal muscle growth in cattle.

Epigenetic Regulation of Adult Myogenesis

Skeletal muscle regeneration is an efficient stem cell-based repair system that ensures healthy musculature. For this repair system to function continuously throughout life, muscle stem cells must contribute to the process of myofiber repair as well as repopulation of the stem cell niche. The decision made by the muscle stem cells to commit to the muscle repair or to remain a stem cell depends upon patterns of gene expression, a process regulated at the epigenetic level. Indeed, it is well accepted that dynamic changes in epigenetic landscapes to control DNA accessibility and expression is a critical component during myogenesis for the effective repair of damaged muscle. Changes in the epigenetic landscape are governed by various posttranslational histone tail modifications, nucleosome repositioning, and DNA methylation events which collectively allow the control of changes in transcription networks during transitions of satellite cells from a dormant quiescent state toward terminal differentiation. This chapter focuses upon the specific epigenetic changes that occur during muscle stem cell-mediated regeneration to ensure myofiber repair and continuity of the stem cell compartment. Furthermore, we explore open questions in the field that are expected to be important areas of exploration as we move toward a more thorough understanding of the epigenetic mechanism regulating muscle regeneration.

Vestigial-like 2 contributes to normal muscle fiber type distribution in mice

Skeletal muscle is composed of heterogeneous populations of myofibers that are classified as slow- and fast-twitch fibers. The muscle fiber-type is regulated in a coordinated fashion by multiple genes, including transcriptional factors and microRNAs (miRNAs). However, players involved in this regulation are not fully elucidated. One of the members of the Vestigial-like factors, Vgll2, is thought to play a pivotal role in TEA domain (TEAD) transcription factor-mediated muscle-specific gene expression because of its restricted expression in skeletal muscles of adult mice. Here, we generated Vgll2 null mice and investigated Vgll2 function in adult skeletal muscles. These mice presented an increased number of fast-twitch type IIb fibers and exhibited a down-regulation of slow type I myosin heavy chain (MyHC) gene, Myh7, which resulted in exercise intolerance. In accordance with the decrease in Myh7, down-regulation of miR-208b, encoded within Myh7 gene and up-regulation of targets of miR-208b, Sox6, Sp3, and Purβ, were observed in Vgll2 deficient mice. Moreover, we detected the physical interaction between Vgll2 and TEAD1/4 in neonatal skeletal muscles. These results suggest that Vgll2 may be both directly and indirectly involved in the programing of slow muscle fibers through the formation of the Vgll2-TEAD complex.

Metabolic control of myofibers: promising therapeutic target for obesity and type 2 diabetes

Mammalian skeletal muscles are composed of two major fibre types (I and II) that differ in terms of size, metabolism and contractile properties. In general, slow-twitch type I fibres are rich in mitochondria and have a greater insulin sensitivity than fast-twitch type II skeletal muscles. Although not widely appreciated, a forced induction of the slow skeletal muscle phenotype may inhibit the progress of obesity and diabetes. This potentially forms the basis for targeting slow/oxidative myofibers in the treatment of obesity. In this context, a better understanding of the molecular basis of fibre-type specification and plasticity may help to identify potential therapeutic targets for obesity and diabetes.

Arginine Methylation by PRMT1 Regulates Muscle Stem Cell Fate

Quiescent muscle stem cells (MSCs) become activated in response to skeletal muscle injury to initiate regeneration. Activated MSCs proliferate and differentiate to repair damaged fibers or self-renew to maintain the pool and ensure future regeneration. The balance between self-renewal, proliferation, and differentiation is a tightly regulated process controlled by a genetic cascade involving determinant transcription factors such as Pax7, Myf5, MyoD, and MyoG. Recently, there have been several reports about the role of arginine methylation as a requirement for epigenetically mediated control of muscle regeneration. Here we report that the protein arginine methyltransferase 1 (PRMT1) is expressed in MSCs and that conditional ablation of PRMT1 in MSCs using Pax7^CreERT2 causes impairment of muscle regeneration. Importantly, PRMT1-deficient MSCs have enhanced cell proliferation after injury but are unable to terminate the myogenic differentiation program, leading to regeneration failure. We identify the coactivator of Six1, Eya1, as a substrate of PRMT1. We show that PRMT1 methylates Eya1 in vitro and that loss of PRMT1 function in vivo prevents Eya1 methylation. Moreover, we observe that PRMT1-deficient MSCs have reduced expression of Eya1/Six1 target MyoD due to disruption of Eya1 recruitment at the MyoD promoter and subsequent Eya1-mediated coactivation. These findings suggest that arginine methylation by PRMT1 regulates muscle stem cell fate through the Eya1/Six1/MyoD axis.

Six1 homeoprotein drives myofiber type IIA specialization in soleus muscle

BACKGROUND

Adult skeletal muscles are composed of slow and fast myofiber subtypes which each express selective genes required for their specific contractile and metabolic activity. Six homeoproteins are transcription factors regulating muscle cell fate through activation of myogenic regulatory factors and driving fast-type gene expression during embryogenesis.

RESULTS

We show here that Six1 protein accumulates more robustly in the nuclei of adult fast-type muscles than in adult slow-type muscles, this specific enrichment takes place during perinatal growth. Deletion of Six1 in soleus impaired fast-type myofiber specialization during perinatal development, resulting in a slow phenotype and a complete lack of Myosin heavy chain 2A (MyHCIIA) expression. Global transcriptomic analysis of wild-type and Six1 mutant myofibers identified the gene networks controlled by Six1 in adult soleus muscle. This analysis showed that Six1 is required for the expression of numerous genes encoding fast-type sarcomeric proteins, glycolytic enzymes and controlling intracellular calcium homeostasis. Parvalbumin, a key player of calcium buffering, in particular, is a direct target of Six1 in the adult myofiber.

CONCLUSIONS

This analysis revealed that Six1 controls distinct aspects of adult muscle physiology in vivo, and acts as a main determinant of fast-fiber type acquisition and maintenance.

MyoD reprogramming requires Six1 and Six4 homeoproteins: genome-wide cis-regulatory module analysis

Myogenic regulatory factors of the MyoD family have the ability to reprogram differentiated cells toward a myogenic fate. In this study, we demonstrate that Six1 or Six4 are required for the reprogramming by MyoD of mouse embryonic fibroblasts (MEFs). Using microarray experiments, we found 761 genes under the control of both Six and MyoD. Using MyoD ChIPseq data and a genome-wide search for Six1/4 MEF3 binding sites, we found significant co-localization of binding sites for MyoD and Six proteins on over a thousand mouse genomic DNA regions. The combination of both datasets yielded 82 genes which are synergistically activated by Six and MyoD, with 96 associated MyoD+MEF3 putative cis-regulatory modules (CRMs). Fourteen out of 19 of the CRMs that we tested demonstrated in Luciferase assays a synergistic action also observed for their cognate gene. We searched putative binding sites on these CRMs using available databases and de novo search of conserved motifs and demonstrated that the Six/MyoD synergistic activation takes place in a feedforward way. It involves the recruitment of these two families of transcription factors to their targets, together with partner transcription factors, encoded by genes that are themselves activated by Six and MyoD, including Mef2, Pbx-Meis and EBF.

Skeletal muscle fiber type: using insights from muscle developmental biology to dissect targets for susceptibility and resistance to muscle disease

Skeletal muscle fibers are classified into fiber types, in particular, slow twitch versus fast twitch. Muscle fiber types are generally defined by the particular myosin heavy chain isoforms that they express, but many other components contribute to a fiber's physiological characteristics. Skeletal muscle fiber type can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength. These findings suggest that some muscle diseases may be treated by shifting fiber type characteristics either from slow to fast, or fast to slow phenotypes, depending on the disease. Recent studies have begun to address which components of muscle fiber types mediate their susceptibility or resistance to muscle disease. However, for many diseases it remains largely unclear why certain fiber types are affected. A substantial body of work has revealed molecular pathways that regulate muscle fiber type plasticity and early developmental muscle fiber identity. For instance, recent studies have revealed many factors that regulate muscle fiber type through modulating the activity of the muscle regulatory transcription factor MYOD1. Future studies of muscle fiber type development in animal models will continue to enhance our understanding of factors and pathways that may provide therapeutic targets to treat muscle diseases. WIREs Dev Biol 2016, 5:518-534. doi: 10.1002/wdev.230 For further resources related to this article, please visit the WIREs website.

Genome-wide association between Six4, MyoD, and the histone demethylase Utx during myogenesis

Adult skeletal muscles can regenerate after injury, due to the presence of satellite cells, a quiescent population of myogenic progenitor cells. Once activated, satellite cells repair the muscle damage by undergoing myogenic differentiation. The myogenic regulatory factors (MRFs) coordinate the process of progenitor differentiation in cooperation with other families of transcription factors (TFs). The Six1 and Six4 homeodomain TFs are expressed in developing and adult muscle and Six1 is critical for embryonic and adult myogenesis. However, the lack of a muscle developmental phenotype in Six4-null mice, which has been attributed to compensation by other Six family members, has discouraged further assessment of the role of Six4 during adult muscle regeneration. By employing genome-wide approaches to address the function of Six4 during adult skeletal myogenesis, we have identified a core set of muscle genes coordinately regulated in adult muscle precursors by Six4 and the MRF MyoD. Throughout the genome of differentiating adult myoblasts, the cooperation between Six4 and MyoD is associated with chromatin repressive mark removal by Utx, a demethylase of histone H3 trimethylated at lysine 27. Among the genes coordinately regulated by Six4 and MyoD are several genes critical for proper in vivo muscle regeneration, implicating a role of Six4 in this process. Using in vivo RNA interference of Six4, we expose an uncompensated function of this TF during muscle regeneration. Together, our results reveal a role for Six4 during adult muscle regeneration and suggest a widespread mechanism of cooperation between Six4 and MyoD.

Contractile and metabolic properties of engineered skeletal muscle derived from slow and fast phenotype mouse muscle

Satellite cells derived from fast and slow muscles have been shown to adopt contractile and metabolic properties of their parent muscle. Mouse muscle shows less distinctive fiber-type profiles than rat or rabbit muscle. Therefore, in this study we sought to determine whether three-dimensional muscle constructs engineered from slow soleus (SOL) and fast tibialis anterior (TA) from mice would adopt the contractile and metabolic properties of their parent muscle. Time-to-peak tension (TPT) and half-relaxation time (1/2RT) was significantly slower in SOL constructs. In agreement with TPT, TA constructs contained significantly higher levels of fast myosin heavy chain (MHC) and fast troponin C, I, and T isoforms. Fast SERCA protein, both slow and fast calsequestrin isoforms and parvalbumin were found at higher levels in TA constructs. SOL constructs were more fatigue resistant and contained higher levels of the mitochondrial proteins SDH and ATP synthase and the fatty acid transporter CPT-1. SOL constructs contained lower levels of the glycolytic enzyme phosphofructokinase but higher levels of the β-oxidation enzymes LCAD and VLCAD suggesting greater fat oxidation. Despite no changes in PGC-1α protein, SOL constructs contained higher levels of SIRT1 and PRC. TA constructs contained higher levels of the slow-fiber program repressor SOX6 and the six transcriptional complex (STC) proteins Eya1 and Six4 which may underlie the higher in fast-fiber and lower slow-fiber program proteins. Overall, we have found that muscles engineered from predominantly slow and fast mouse muscle retain contractile and metabolic properties of their native muscle.

The effect of serum origin on tissue engineered skeletal muscle function

Skeletal muscle phenotype is regulated by a complex interaction between genetic, hormonal, and electrical inputs. However, because of the interrelatedness of these factors in vivo it is difficult to determine the importance of one over the other. Over the last 5 years, we have engineered skeletal muscles in the European Union (EU) and the United States (US) using the same clone of C2C12 cells. Strikingly, the dynamics of contraction of the muscles was dramatically different. Therefore, in this study we sought to determine whether the hormonal milieu (source of fetal bovine serum (FBS)) could alter engineered muscle phenotype. In muscles engineered in serum of US origin time-to-peak tension (2.2-fold), half relaxation (2.6-fold), and fatigue resistance (improved 25%) all showed indications of a shift towards a slower phenotype. Even though there was a dramatic shift in the rate of contraction, myosin heavy chain expression was the same. The contraction speed was instead related to a shift in calcium release/sensitivity proteins (DHPR = 3.1-fold lower, slow CSQ = 3.4-fold higher, and slow TnT = 2.4-fold higher) and calcium uptake proteins (slow SERCA = 1.7-fold higher and parvalbumin = 41-fold lower). These shifts in calcium dynamics were accompanied by a partial shift in metabolic enzymes, but could not be explained by purported regulators of muscle phenotype. These data suggest that hormonal differences in serum of USDA and EU origin cause a shift in calcium handling resulting in a dramatic change in engineered muscle function.

Function and position determine relative proportions of different fiber types in limb muscles of the lizard Tropidurus psammonastes

Skeletal muscles can be classified as flexors or extensors according to their function, and as dorsal or ventral according to their position. The latter classification evokes their embryological origin from muscle masses initially divided during limb development, and muscles sharing a given position do not necessarily perform the same function. Here, we compare the relative proportions of different fiber types among six limb muscles in the lizard Tropidurus psammonastes. Individual fibers were classified as slow oxidative (SO), fast glycolytic (FG) or fast oxidative-glycolytic (FOG) based on mitochondrial content; muscles were classified according to position and function. Mixed linear models considering one or both effects were compared using likelihood ratio tests. Variation in the proportion of FG and FOG fibers is mainly explained by function (flexor muscles have on average lower proportions of FG and higher proportions of FOG fibers), while variation in SO fibers is better explained by position (they are less abundant in ventral muscles than in those developed from a dorsal muscle mass). Our results clarify the roles of position and function in determining the relative proportions of the various muscle fibers and provide evidence that these factors may differentially affect distinct fiber types.

The role of Six1 in the genesis of muscle cell and skeletal muscle development

The sine oculis homeobox 1 (Six1) gene encodes an evolutionarily conserved transcription factor. In the past two decades, much research has indicated that Six1 is a powerful regulator participating in skeletal muscle development. In this review, we summarized the discovery and structural characteristics of Six1 gene, and discussed the functional roles and molecular mechanisms of Six1 in myogenesis and in the formation of skeletal muscle fibers. Finally, we proposed areas of future interest for understanding Six1 gene function.

Loss of Prox1 in striated muscle causes slow to fast skeletal muscle fiber conversion and dilated cardiomyopathy

Correct regulation of troponin and myosin contractile protein gene isoforms is a critical determinant of cardiac and skeletal striated muscle development and function, with misexpression frequently associated with impaired contractility or disease. Here we reveal a novel requirement for Prospero-related homeobox factor 1 (Prox1) during mouse heart development in the direct transcriptional repression of the fast-twitch skeletal muscle genes troponin T3, troponin I2, and myosin light chain 1. A proportion of cardiac-specific Prox1 knockout mice survive beyond birth with hearts characterized by marked overexpression of fast-twitch genes and postnatal development of a fatal dilated cardiomyopathy. Through conditional knockout of Prox1 from skeletal muscle, we demonstrate a conserved requirement for Prox1 in the repression of troponin T3, troponin I2, and myosin light chain 1 between cardiac and slow-twitch skeletal muscle and establish Prox1 ablation as sufficient to cause a switch from a slow- to fast-twitch muscle phenotype. Our study identifies conserved roles for Prox1 between cardiac and skeletal muscle, specifically implicated in slow-twitch fiber-type specification, function, and cardiomyopathic disease.

Molecular ties between the cell cycle and differentiation in embryonic stem cells

Attainment of the differentiated state during the final stages of somatic cell differentiation is closely tied to cell cycle progression. Much less is known about the role of the cell cycle at very early stages of embryonic development. Here, we show that molecular pathways involving the cell cycle can be engineered to strongly affect embryonic stem cell differentiation at early stages in vitro. Strategies based on perturbing these pathways can shorten the rate and simplify the lineage path of ES differentiation. These results make it likely that pathways involving cell proliferation intersect at various points with pathways that regulate cell lineages in embryos and demonstrate that this knowledge can be used profitably to guide the path and effectiveness of cell differentiation of pluripotent cells.

Six homeoproteins and a Iinc-RNA at the fast MYH locus lock fast myofiber terminal phenotype

Thousands of long intergenic non-coding RNAs (lincRNAs) are encoded by the mammalian genome. However, the function of most of these lincRNAs has not been identified in vivo. Here, we demonstrate a role for a novel lincRNA, linc-MYH, in adult fast-type myofiber specialization. Fast myosin heavy chain (MYH) genes and linc-MYH share a common enhancer, located in the fast MYH gene locus and regulated by Six1 homeoproteins. linc-MYH in nuclei of fast-type myofibers prevents slow-type and enhances fast-type gene expression. Functional fast-sarcomeric unit formation is achieved by the coordinate expression of fast MYHs and linc-MYH, under the control of a common Six-bound enhancer.

Gene regulatory networks and transcriptional mechanisms that control myogenesis

We discuss the upstream regulators of myogenesis that lead to the activation of myogenic determination genes and subsequent differentiation, focusing on the mouse model. Key upstream genes, such as Pax3 and Pax7, Six1 and Six4, or Pitx2, participate in gene regulatory networks at different sites of skeletal muscle formation. MicroRNAs also intervene, with emerging evidence for the role of other noncoding RNAs. Myogenic determination and subsequent differentiation depend on members of the MyoD family. We discuss new insights into mechanisms underlying the transcriptional activity of these factors.

Crucial transcription factors in tendon development and differentiation: their potential for tendon regeneration

Tendons that connect muscles to bone are often the targets of sports injuries. The currently unsatisfactory state of tendon repair is largely attributable to the limited understanding of basic tendon biology. A number of tendon lineage-related transcription factors have recently been uncovered and provide clues for the better understanding of tendon development. Scleraxis and Mohawk have been identified as critical transcription factors in tendon development and differentiation. Other transcription factors, such as Sox9 and Egr1/2, have also been recently reported to be involved in tendon development. However, the molecular mechanisms and application of these transcription factors remain largely unclear and this prohibits their use in tendon therapy. Here, we systematically review and analyze recent findings and our own data concerning tendon transcription factors and tendon regeneration. Based on these findings, we provide interaction and temporal programming maps of transcription factors, as a basis for future tendon therapy. Finally, we discuss future directions for tendon regeneration with differentiation and trans-differentiation approaches based on transcription factors.

Comparative myogenesis in teleosts and mammals

Skeletal myogenesis has been and is currently under extensive study in both mammals and teleosts, with the latter providing a good model for skeletal myogenesis because of their flexible and conserved genome. Parallel investigations of muscle studies using both these models have strongly accelerated the advances in the field. However, when transferring the knowledge from one model to the other, it is important to take into account both their similarities and differences. The main difficulties in comparing mammals and teleosts arise from their different temporal development. Conserved aspects can be seen for muscle developmental origin and segmentation, and for the presence of multiple myogenic waves. Among the divergences, many fish have an indeterminate growth capacity throughout their entire life span, which is absent in mammals, thus implying different post-natal growth mechanisms. This review covers the current state of the art on myogenesis, with a focus on the most conserved and divergent aspects between mammals and teleosts.

Novel polymorphisms of SIX4 gene and their association with body measurement traits in Qinchuan cattle

Sine oculis homeobox homolog 4 (SIX4) gene belongs to the sine oculis/SIX gene family, which includes six members in vertebrates. SIX4 gene plays a crucial role in skeletal myogenesis, and its genetic variations or deficiency may cause hypopituitarism, suggesting that SIX4 gene is a potential candidate gene affecting body measurement traits (BMTs) in animals. Herein, the objectives of this study were to identify genetic polymorphisms of bovine SIX4 gene and to analyze potential association between single nucleotide polymorphisms (SNPs) and body measurement traits in Qinchuan cattle. In the present study, we investigated polymorphisms of SIX4 gene in 426 Qinchuan cattle using DNA sequencing and polymerase chain reaction-restriction fragment length polymorphisms. Three novel SNPs were identified within bovine SIX4 gene. Associations between body measurement traits and SIX4 gene polymorphisms were investigated, and significant statistical associations were found between polymorphisms of these three SNPs and body measurement traits (P<0.05). Hence, based on results obtained from this study, we conjectured that SIX4 gene may have potential effects on body measurement traits in Qinchuan cattle population and could be used for marker-assisted selection.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Transcriptional regulation and alternative splicing cooperate in muscle fiber-type specification in flies and mammals

Muscles coordinate body movements throughout the animal kingdom. Each skeletal muscle is built of large, multi-nucleated cells, called myofibers, which are classified into several functionally distinct types. The typical fiber-type composition of each muscle arises during development, and in mammals is extensively adjusted in response to postnatal exercise. Understanding how functionally distinct muscle fiber-types arise is important for unraveling the molecular basis of diseases from cardiomyopathies to muscular dystrophies. In this review, we focus on recent advances in Drosophila and mammals in understanding how muscle fiber-type specification is controlled by the regulation of transcription and alternative splicing. We illustrate the cooperation of general myogenic transcription factors with muscle fiber-type specific transcriptional regulators as a basic principle for fiber-type specification, which is conserved from flies to mammals. We also examine how regulated alternative splicing of sarcomeric proteins in both flies and mammals can directly instruct the physiological and biophysical differences between fiber-types. Thus, research in Drosophila can provide important mechanistic insight into muscle fiber specification, which is relevant to homologous processes in mammals and to the pathology of muscle diseases.

Gene Networks during Skeletal Myogenesis

Mammalian skeletal muscles are derived from mesoderm segments flanking the embryonic midline. Upon receiving inductive cues from the adjacent neural tube, lateral plate mesoderm, and surface ectoderm, muscle precursors start to delaminate, migrate to their final destinations and proliferate. Muscle precursor cells become committed to the myogenic fate, become differentiated muscle cells, and fuse to form myofibers. Myofibers then fuse together to form the muscle groups. Muscle precursor cells have the ability to proliferate, and differentiate during development, while a subset remains capable of regeneration and repair of local injuries in adulthood. When the process of muscle development is perturbed such as in muscular dystrophies and injuries, ways to intervene and allow for proper muscle development or repair are the focus of regenerative medicine. Thus, understanding the developmental program of muscle at the genetic, cellular, and molecular levels has become a major focus of skeletal muscle regeneration research in the last few years.