MyoD-expressing progenitors are essential for skeletal myogenesis and satellite cell development

Effects of 3-(4-Hydroxy-3-methoxyphenyl)propionic Acid on Enhancing Grip Strength and Inhibiting Protein Catabolism Induced by Exhaustive Exercise

3-(4-Hydroxy-3-methoxyphenyl)propionic acid (HMPA), also known as dihydroferulic acid, is a hydroxycinnamic acid derivative that can be derived from the microbial transformation of dietary polyphenols or naturally obtained from fermented foods. Although numerous studies have documented its antioxidant and anti-obesity effects, the effect of HMPA on muscle function remains unknown. This study investigated the effects of HMPA on muscle strength and exercise endurance capacity. Mice were orally administered low and high doses of HMPA for 14 days and subjected to grip force and treadmill exhaustion tests to evaluate muscle function. Our results showed that HMPA-administered groups significantly enhanced absolute grip strength (p = 0.0256) and relative grip strength (p = 0.0209), and low-dose HMPA decreased the plasma level of blood urea nitrogen after exercise (p = 0.0183), but HMPA did not affect endurance performance. Low-dose HMPA administration increased Myf5 expression in sedentary mice (p = 0.0106), suggesting that low-dose HMPA may promote muscle development. Additionally, HMPA improved hepatic glucose and lipid metabolism, and inhibited muscular lipid metabolism and protein catabolism, as indicated by changes in mRNA expression levels of related genes. These findings suggest that HMPA may be a promising dietary supplement for muscle health and performance.

Cardiac commitment driven by MyoD expression in pericardial stem cells

Cellular therapy holds immense promise to remuscularize the damaged myocardium but is practically hindered by limited allogeneic sources of cardiac-committed cells that engraft stably in the recipient heart after transplantation. Here, we demonstrate that the pericardial tissue harbors myogenic stem cells (pSCs) that are activated in response to inflammatory signaling after myocardial infarction (MI). The pSCs derived from the MI rats (MI-pSCs) show in vivo and in vitro cardiac commitment characterized by cardiac-specific Tnnt2 expression and formation of rhythmic contraction in culture. Bulk RNA-seq analysis reveals significant upregulation of a panel of genes related to cardiac/myogenic differentiation, paracrine factors, and extracellular matrix in the activated pSCs compared to the control pSCs (Sham-pSCs). Notably, we define MyoD as a key factor that governs the process of cardiac commitment, as siRNA-mediated MyoD gene silencing results in a significant reduction of myogenic potential. Injection of the cardiac-committed cells into the infarcted rat heart leads to long-term survival and stable engraftment in the recipient myocardium. Therefore, these findings point to pericardial myogenic progenitors as an attractive candidate for cardiac cell-based therapy to remuscularize the damaged myocardium.

Effects of glycyrrhiza polysaccharides on growth performance, meat quality, serum parameters and growth/meat quality-related gene expression in broilers

With growing restrictions on the use of antibiotics in animal feed, plant extracts are increasingly favored as natural feed additive sources. Glycyrrhiza polysaccharide (GP), known for its multifaceted biological benefits including growth promotion, immune enhancement, and antioxidative properties, has been the focus of recent studies. Yet, the effects and mechanisms of GP on broiler growth and meat quality remain to be fully elucidated. This study aimed to investigate the effects of GP on growth, serum biochemistry, meat quality, and gene expression in broilers. The broilers were divided into five groups, each consisting of five replicates with six birds. These groups were supplemented with 0, 500, 1,000, 1,500, and 2,000 mg/kg of GP in their basal diets, respectively, for a period of 42 days. The results indicated that from day 22 to day 42, and throughout the entire experimental period from day 1 to day 42, the groups receiving 1,000 and 1,500 mg/kg of GP showed a significant reduction in the feed-to-gain ratio (F:G) compared to the control group. On day 42, an increase in serum growth hormone (GH) levels was shown in groups supplemented with 1,000 mg/kg GP or higher, along with a significant linear increase in insulin-like growth factor-1 (IGF-1) concentration. Additionally, significant upregulation of GH and IGF-1 mRNA expression levels was noted in the 1,000 and 1,500 mg/kg GP groups. Furthermore, GP significantly elevated serum concentrations of alkaline phosphatase (AKP) and globulin (GLB) while reducing blood urea nitrogen (BUN) levels. In terms of meat quality, the 1,500 and 2,000 mg/kg GP groups significantly increased fiber density in pectoral muscles and reduced thiobarbituric acid (TBA) content. GP also significantly decreased cooking loss rate in both pectoral and leg muscles and the drip loss rate in leg muscles. It increased levels of linoleic acid and oleic acid, while decreasing concentrations of stearic acid, myristic acid, and docosahexaenoic acid. Finally, the study demonstrated that the 1,500 mg/kg GP group significantly enhanced the expression of myogenin (MyoG) and myogenic differentiation (MyoD) mRNA in leg muscles. Overall, the study determined that the optimal dosage of GP in broiler feed is 1,500 mg/kg.

Current understanding and future perspectives on the extraction, structures, and regulation of muscle function of tea pigments

With the aggravating aging of modern society, the sarcopenia-based aging syndrome poses a serious potential threat to the health of the elderly. Natural dietary supplements show great potential to reduce muscle wasting and enhance muscle performance. Tea has been widely recognized for its health-promoting effects. which contains active ingredients such as tea polyphenols, tea pigments, tea polysaccharides, theanine, caffeine, and vitamins. In different tea production processes, the oxidative condensation and microbial transformation of catechins and other natural substances from tea promotes the production of various tea pigments, including theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs). Tea pigments have shown a positive effect on maintaining muscle health. Nevertheless, the relationship between tea pigments and skeletal muscle function has not been comprehensively elucidated. In addition, the numerous research on the extraction and purification of tea pigments is disordered with the limited recent progress due to the complexity of species and molecular structure. In this review, we sort out the strategies for the separation of tea pigments, and discuss the structures of tea pigments. On this basis, the regulation mechanisms of tea pigments on muscle functional were emphasized. This review highlights the current understanding on the extraction methods, molecular structures and regulation mechanisms of muscle function of tea pigments. Furthermore, main limitations and future perspectives are proposed to provide new insights into broadening theoretical research and industrial applications of tea pigments in the future.

Androgen action on myogenesis throughout the lifespan; comparison with neurogenesis

Androgens' pleiotropic actions in promoting sex differences present not only a challenge to providing a comprehensive account of their function, but also an opportunity to gain insights by comparing androgenic actions across organ systems. Although often overlooked by neuroscientists, skeletal muscle is another androgen-responsive organ system which shares with the nervous system properties of electrochemical excitability, behavioral relevance, and remarkable capacity for adaptive plasticity. Here we review androgenic regulation of mitogenic plasticity in skeletal muscle with the goal of identifying areas of interest to those researching androgenic mechanisms mediating sexual differentiation of neurogenesis. We use an organizational-activational framework to relate broad areas of similarity and difference between androgen effects on mitogenesis in muscle and brain throughout the lifespan, from early organogenesis, through pubertal organization, adult activation, and aging. The focus of the review is androgenic regulation of muscle-specific stem cells (satellite cells), which share with neural stem cells essential functions in development, plasticity, and repair, albeit with distinct, muscle-specific features. Also considered are areas of paracrine and endocrine interaction between androgen action on muscle and nervous system, including mediation of neural plasticity of innervating and distal neural populations by muscle-produced trophic factors.

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.

Molecular clocks, satellite cells, and skeletal muscle regeneration

Skeletal muscle comprises approximately 50% of individual body mass and plays vital roles in locomotion, heat production, and whole body metabolic homeostasis. This tissue exhibits a robust diurnal rhythm that is under control of the suprachiasmatic nucleus (SCN) region of the hypothalamus. The SCN acts as a "central" coordinator of circadian rhythms, while cell-autonomous "peripheral" clocks are located within almost all other tissues/organs in the body. Synchronization of the peripheral clocks in muscles (and other tissues) together with the central clock is crucial to ensure temporally coordinated physiology across all organ systems. By virtue of its mass, human skeletal muscle contains the largest collection of peripheral clocks, but within muscle resides a local stem cell population, satellite cells (SCs), which have their own functional molecular clock, independent of the numerous muscle clocks. Skeletal muscle has a daily turnover rate of 1%-2%, so the regenerative capacity of this tissue is important for whole body homeostasis/repair and depends on successful SC myogenic progression (i.e., proliferation, differentiation, and fusion). Emerging evidence suggests that SC-mediated muscle regeneration may, in part, be regulated by molecular clocks involved in SC-specific diurnal transcription. Here we provide insights on molecular clock regulation of muscle regeneration/repair and provide a novel perspective on the interplay between SC-specific molecular clocks, myogenic programs, and cell cycle kinetics that underpin myogenic progression.

Generation of a MyoD knock-in reporter mouse line to study muscle stem cell dynamics and heterogeneity

Myoblast determination protein 1 (MyoD) dynamics define the activation status of muscle stem cells (MuSCs), aiding in muscle tissue regeneration after injury. However, the lack of experimental platforms to monitor MyoD dynamics in vitro and in vivo has hampered the investigation of fate determination and heterogeneity of MuSCs. Herein, we report a MyoD knock-in (MyoD-KI) reporter mouse expressing tdTomato at the endogenous MyoD locus. Expression of tdTomato in MyoD-KI mice recapitulated the endogenous MyoD expression dynamics in vitro and during the early phase of regeneration in vivo. Additionally, we showed that tdTomato fluorescence intensity defines MuSC activation status without immunostaining. Based on these features, we developed a high-throughput screening system to assess the effects of drugs on the behavior of MuSCs in vitro. Thus, MyoD-KI mice are an invaluable resource for studying the dynamics of MuSCs, including their fate decisions and heterogeneity, and for drug screening in stem cell therapy.

Digits in a dish: An in vitro system to assess the molecular genetics of hand/foot development at single-cell resolution

In vitro models allow for the study of developmental processes outside of the embryo. To gain access to the cells mediating digit and joint development, we identified a unique property of undifferentiated mesenchyme isolated from the distal early autopod to autonomously re-assemble forming multiple autopod structures including: digits, interdigital tissues, joints, muscles and tendons. Single-cell transcriptomic analysis of these developing structures revealed distinct cell clusters that express canonical markers of distal limb development including: Col2a1, Col10a1, and Sp7 (phalanx formation), Thbs2 and Col1a1 (perichondrium), Gdf5, Wnt5a, and Jun (joint interzone), Aldh1a2 and Msx1 (interdigital tissues), Myod1 (muscle progenitors), Prg4 (articular perichondrium/articular cartilage), and Scx and Tnmd (tenocytes/tendons). Analysis of the gene expression patterns for these signature genes indicates that developmental timing and tissue-specific localization were also recapitulated in a manner similar to the initiation and maturation of the developing murine autopod. Finally, the in vitro digit system also recapitulates congenital malformations associated with genetic mutations as in vitro cultures of Hoxa13 mutant mesenchyme produced defects present in Hoxa13 mutant autopods including digit fusions, reduced phalangeal segment numbers, and poor mesenchymal condensation. These findings demonstrate the robustness of the in vitro digit system to recapitulate digit and joint development. As an in vitro model of murine digit and joint development, this innovative system will provide access to the developing limb tissues facilitating studies to discern how digit and articular joint formation is initiated and how undifferentiated mesenchyme is patterned to establish individual digit morphologies. The in vitro digit system also provides a platform to rapidly evaluate treatments aimed at stimulating the repair or regeneration of mammalian digits impacted by congenital malformation, injury, or disease.

Tracing the developmental origin of tissue stem cells

Tissue stem cells are vital for organ homeostasis and regeneration owing to their ability to self-renew and differentiate into the various cell types that constitute organ tissue. These stem cells are formed during complex and dynamic organ development, necessitating spatial-temporal coordination of morphogenetic events and cell fate specification during this process. In recent years, technological advances have enabled the tracing of the cellular dynamics, states, and lineages of individual cells over time in relation to tissue morphological changes. These dynamic data have not only revealed the origin of tissue stem cells in various organs but have also led to an understanding of the molecular, cellular, and biophysical bases of tissue stem cell formation. Herein, we summarize recent findings on the developmental origin of tissue stem cells in the hair follicles, intestines, brain, skeletal muscles, and hematopoietic system, and further discuss how stem cell fate specification is coordinated with tissue topology.

Exploring high-resolution chromatin interaction changes and functional enhancers of myogenic marker genes during myogenic differentiation

Skeletal muscle differentiation (myogenesis) is a complex and highly coordinated biological process regulated by a series of myogenic marker genes. Chromatin interactions between gene’s promoters and their enhancers have an important role in transcriptional control. However, the high-resolution chromatin interactions of myogenic genes and their functional enhancers during myogenesis remain largely unclear. Here, we used circularized chromosome conformation capture coupled with next generation sequencing (4C-seq) to investigate eight myogenic marker genes in C2C12 myoblasts (C2C12-MBs) and C2C12 myotubes (C2C12-MTs). We revealed dynamic chromatin interactions of these marker genes during differentiation and identified 163 and 314 significant interaction sites (SISs) in C2C12-MBs and C2C12-MTs, respectively. The interacting genes of SISs in C2C12-MTs were mainly involved in muscle development, and histone modifications of the SISs changed during differentiation. Through functional genomic screening, we also identified 25 and 41 putative active enhancers in C2C12-MBs and C2C12-MTs, respectively. Using luciferase reporter assays for putative enhancers of Myog and Myh3, we identified eight activating enhancers. Furthermore, dCas9-KRAB epigenome editing and RNA-Seq revealed a role for Myog enhancers in the regulation of Myog expression and myogenic differentiation in the native genomic context. Taken together, this study lays the groundwork for understanding 3D chromatin interaction changes of myogenic genes during myogenesis and provides insights that contribute to our understanding of the role of enhancers in regulating myogenesis.

Comparative development of limb musculature in phylogenetically and ecologically divergent lizards

BACKGROUND

Squamate reptiles (lizards, snakes, and amphisbaenians) exhibit incredible diversity in their locomotion, behavior, morphology, and ecological breadth. Although they often are used as models of locomotor diversity, surprisingly little attention has been given to muscle development in squamate reptiles. In fact, the most detailed examination was conducted almost 80 years ago and solely focused on the proximal limb regions. Herein, we present forelimb and hindlimb muscle morphogenesis data for three lizard species with different locomotion and feeding strategies: the desert grassland whiptail lizard, the central bearded dragon, and the veiled chameleon. This study fills critical gaps in our understanding of muscle morphogenesis in squamate reptiles and presents a comparative and temporospatial analysis of muscle development.

RESULTS

Our results reveal a conserved pattern of early muscle development among lizards with different adult morphologies and ecologies. The variations that exist are concentrated in distal regions, particularly the specialized autopodia of chameleons, where differentiation of muscles associated with the digits is delayed.

CONCLUSIONS

The chameleon autopod provides an example of major evolutionary modifications to the skeleton with only minor disruption of the conserved order and pattern of limb muscle development. This robustness of muscle patterning facilitates the evolution of extreme yet functional phenotypes.

Decorin regulates myostatin and enhances proliferation and differentiation of embryonic myoblasts in Leizhou black duck

Skeletal muscle is one of the most important economic traits in the poultry industry whose development goes through several processes influenced by several candidate genes. This study explored the regulatory role of DCN on MSTN and the influence of these genes on the proliferation and differentiation of embryonic myoblasts in Leizhou black ducks. Embryonic myoblasts were transfected with over-expressing DCN, Si-DCN, and empty vector and cultured for 24 h, 48 h, and 72 h of proliferation and the comparative expression of DCN and MSTN were measured. The results showed that cells transfected with the over-expression DCN had a significantly (P < 0.05) higher expression of DCN mRNA than the normal group and the expression of MSTN mRNA showed a downward trend during the proliferation of myoblasts. DCN mRNA expression was lower in cells transfected with Si-DCN than the normal group in all stages of proliferation. While the expression of MSTN in the Si-DCN transfected group was higher than the normal group with a significant (P < 0.05) difference at the 72 h stage. DCN mRNA increased at the early stage of differentiation but decreased (P > 0.05) from the 6th day to the 8th day of differentiation. The level of MSTN increased gradually during the differentiation process of myoblasts until it decreased significantly on the 8th day. These results show that DCN enhances the proliferation and differentiation of Leizhou black duck myoblasts and suppresses MSTN activity.

Low-Frequency Electrical Stimulation Promotes Satellite Cell Activities to Facilitate Muscle Regeneration at an Early Phase in a Rat Model of Muscle Strain

The capability of regeneration for skeletal muscle after injury depends on the differentiation and proliferation ability of the resident stem cells called satellite cells. It has been reported that electrical stimulation was widely used in clinical conditions to facilitate muscle regeneration after injury, but the characterization of satellite cell responses to the context of low-frequency electrical stimulation in early-phase muscle strain conditions has not been fully clarified. In this study, we aim to investigate the effects of low-frequency electrical stimulation (frequency: 20 Hz; duration: 30 minutes, twice daily) on satellite cell activities in a rat model for the early phase of muscle strain. Firstly, we adopted our previously developed rat model to mimic the early phase of muscle strain in human. After then, we examined the effects of low-frequency electrical stimulation on histopathological changes of the muscle fiber by hematoxylin and eosin (H&E) staining. Finally, we investigated the effects of low-frequency electrical stimulation on satellite cell proliferation and differentiation by quantification of the expression level of the specific proteins using western blot analyses. The muscle strain in biceps femoris muscles of rats can be induced by high-speed rotation from knee flexion 50° to full knee extension at 960°·s⁻¹ angular velocity during its tetany by activating the sciatic nerve, as evidenced by a widening of the interstitial space between fibers, and more edema or necrosis fibers were detected in the model rats without treatment than in control rats. After treatment with low-frequency electrical stimulation (frequency: 20 Hz; duration: 30 minutes, twice daily), the acute strained biceps femoris muscles of rats showed obvious improvement of histomorphology as indicated by more mature muscle fibers with well-ordered formation with clear boundaries. Consistently, the expression levels of the MyoD and myogenin were marked higher than those in the rats in the animal model group, indicating increased satellite cell proliferating and differentiating activities by low-frequency electrical stimulation. This study shows that low-frequency electrical stimulation provides an effective stimulus to upregulate the protein expression of MyoD/myogenin and accelerate the restoration of structure during the early phase of muscle strain. This may have significance for clinical practice. Optimization of low-frequency electrical stimulation parameters may enhance the therapeutic outcome in patients.

Genome-wide analysis of chromatin structure changes upon MyoD binding in proliferative myoblasts during the cell cycle

MyoD, a myogenic differentiation protein, has been studied for its critical role in skeletal muscle differentiation. MyoD-expressing myoblasts have a potency to be differentiated with proliferation of ectopic cells. However, little is known about the effect on chromatin structure of MyoD binding in proliferative myoblasts. In this study, we evaluated the chromatin structure around MyoD-bound genome regions during the cell cycle by chromatin immunoprecipitation sequencing. Genome-wide analysis of histone modifications was performed in proliferative mouse C2C12 myoblasts during three phases (G1, S, G2/M) of the cell cycle. We found that MyoD-bound genome regions had elevated levels of active histone modifications, such as H3K4me1/2/3, and H3K27ac, compared with MyoD-unbound genome regions during the cell cycle. We also demonstrated that the elevated H3K4me2/3 modification level was maintained during the cell cycle, whereas the H3K27ac and H3K4me1 modification levels decreased to the same level as MyoD-unbound genome regions during the later phases. Immunoblot analysis revealed that MyoD abundance was high in the G1 phase then decreased in the S and G2/M phases. Our results suggest that MyoD binding formed selective epigenetic memories with H3K4me2/3 during the cell cycle in addition to myogenic gene induction via active chromatin formation coupled with transcription.

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.

Local retinoic acid signaling directs emergence of the extraocular muscle functional unit

Coordinated development of muscles, tendons, and their attachment sites ensures emergence of functional musculoskeletal units that are adapted to diverse anatomical demands among different species. How these different tissues are patterned and functionally assembled during embryogenesis is poorly understood. Here, we investigated the morphogenesis of extraocular muscles (EOMs), an evolutionary conserved cranial muscle group that is crucial for the coordinated movement of the eyeballs and for visual acuity. By means of lineage analysis, we redefined the cellular origins of periocular connective tissues interacting with the EOMs, which do not arise exclusively from neural crest mesenchyme as previously thought. Using 3D imaging approaches, we established an integrative blueprint for the EOM functional unit. By doing so, we identified a developmental time window in which individual EOMs emerge from a unique muscle anlage and establish insertions in the sclera, which sets these muscles apart from classical muscle-to-bone type of insertions. Further, we demonstrate that the eyeballs are a source of diffusible all-trans retinoic acid (ATRA) that allow their targeting by the EOMs in a temporal and dose-dependent manner. Using genetically modified mice and inhibitor treatments, we find that endogenous local variations in the concentration of retinoids contribute to the establishment of tendon condensations and attachment sites that precede the initiation of muscle patterning. Collectively, our results highlight how global and site-specific programs are deployed for the assembly of muscle functional units with precise definition of muscle shapes and topographical wiring of their tendon attachments.

Distinct Phases of Postnatal Skeletal Muscle Growth Govern the Progressive Establishment of Muscle Stem Cell Quiescence

Muscle stem cells (or muscle satellite cells [MuSCs]) are required for postnatal growth. Yet, the detailed characterization of myogenic progression and establishment of quiescence during this process remains poorly documented. Here, we provide an overview of myogenic cells heterogeneity and dynamic from birth to adulthood using flow cytometry. We demonstrated that PAX7+ cells acquire an increasing ability to progress in the myogenic program from birth to adulthood. We then simultaneously analyzed the cycling state (KI67 expression) of the MuSCs and progenitors (PAX7+) and their progression into myogenic precursors (PAX7−MYOD+) and differentiating cells (MYOG+) in vivo. We identified two distinct peaks of myogenic differentiation between P7–P10 and P21–P28, and showed that the quiescent MuSC pool is established between 7 and 8 weeks of age. Overall our study provides a comprehensive in vivo characterization of myogenic heterogeneity and demonstrates the highly dynamic nature of skeletal muscle postnatal growth process.

•

Quiescent MuSC pool is fully established between 7 and 8 weeks after birth

•

Postnatal growth comprises two successive phases of differentiation

•

Identification of the complex cellular heterogeneity during myogenic progression

•

Redefinition of the progression of myogenic cells into the differentiation program

Mechanism and Functions of Identified miRNAs in Poultry Skeletal Muscle Development – A Review

Development of the skeletal muscle goes through several complex processes regulated by numerous genetic factors. Although much efforts have been made to understand the mechanisms involved in increased muscle yield, little work is done about the miRNAs and candidate genes that are involved in the skeletal muscle development in poultry. Comprehensive research of candidate genes and single nucleotide related to poultry muscle growth is yet to be experimentally unraveled. However, over a few periods, studies in miRNA have disclosed that they actively participate in muscle formation, differentiation, and determination in poultry. Specifically, miR-1, miR-133, and miR-206 influence tissue development, and they are highly expressed in the skeletal muscles. Candidate genes such as CEBPB, MUSTN1, MSTN, IGF1, FOXO3, mTOR, and NFKB1, have also been identified to express in the poultry skeletal muscles development. However, further researches, analysis, and comprehensive studies should be made on the various miRNAs and gene regulatory factors that influence the skeletal muscle development in poultry. The objective of this review is to summarize recent knowledge in miRNAs and their mode of action as well as transcription and candidate genes identified to regulate poultry skeletal muscle development.

Sinensetin regulates age-related sarcopenia in cultured primary thigh and calf muscle cells

Background

Sarcopenia, the decline of skeletal muscle tissue attributed to primary aging is a major concern in older adults. Flavonoids might have potential benefits by modulating the regulation of satellite cells, thus preventing muscle loss. Sinensetin (SIN), a citrus methylated flavone with anti-inflammatory and anti-proliferative activity, can enhance lipolysis. The objective of the present study was to investigate whether SIN might have sarcopenia-suppressing effect on satellite cells from thigh and calf muscle tissues of young and old rats.

Methods

Primary muscle cells were obtained from thigh and calf tissues of young and old group rats by dissection. Obtained satellite cells were incubated with indicated concentrations of SIN (50 and 100 μM) treated and untreated condition in differentiation medium. Morphological changes of cells were examined using a phase-contrast microscope. Protein expression levels of myoD and myogenin were analyzed by Western blot. Cells treated with or without SIN under differentiation condition were also immunocytochemically stained for myogenin and 4′,6-diamidino-2-phenylindole (DAPI).

Results

Morphologically, the differentiation extracted satellite cells was found to be more evident in SIN treated group of aged rat′s cells than that in SIN untreated group. Expression levels of myoD and myogenin proteins involved in myogenesis were increased upon treatment with SIN.

Conclusions

Collectively, our results indicate that SIN can alleviate age-related sarcopenia by increasing differentiation rate and protein levels of myoD and myogenin.

The bHLH Protein Nulp1 is Essential for Femur Development Via Acting as a Cofactor in Wnt Signaling in Drosophila

Background:

The basic helix-loop-helix (bHLH) protein families are a large class of transcription factors, which are associated with cell proliferation, tissue differentiation, and other important development processes. We reported that the Nuclear localized protein-1 (Nulp1) might act as a novel bHLH transcriptional factor to mediate cellular functions. However, its role in development in vivo remains unknown.

Methods:

Nulp1 (dNulp1) mutants are generated by CRISPR/Cas9 targeting the Domain of Unknown Function (DUF654) in its C terminal. Expression of Wg target genes are analyzed by qRT-PCR. We use the Top-Flash luciferase reporter assay to response to Wg signaling.

Results:

Here we show that Drosophila Nulp1 (dNulp1) mutants, generated by CRISPR/Cas9 targeting the Domain of Unknown Function (DUF654) in its C terminal, are partially homozygous lethal and the rare escapers have bent femurs, which are similar to the major manifestation of congenital bent-bone dysplasia in human Stuve-Weidemann syndrome. The fly phenotype can be rescued by dNulp1 over-expression, indicating that dNulp1 is essential for fly femur development and survival. Moreover, dNulp1 overexpression suppresses the notch wing phenotype caused by the overexpression of sgg/GSK3β, an inhibitor of the canonical Wnt cascade. Furthermore, qRT-PCR analyses show that seven target genes positively regulated by Wg signaling pathway are down-regulated in response to dNulp1 knockout, while two negatively regulated Wg targets are up-regulated in dNulp1 mutants. Finally, dNulp1 overexpression significantly activates the Top-Flash Wnt signaling reporter.

Conclusion:

We conclude that bHLH protein dNulp1 is essential for femur development and survival in Drosophila by acting as a positive cofactor in Wnt/Wingless signaling.

Expression analysis of limb element markers during mouse embryonic development

Background: While data regarding expression of limb element and tissue markers during normal mouse limb development exist, few studies show expression patterns in upper and lower limbs throughout key limb development stages. A comparison to normal developmental events is essential when analyzing development of the limb in mutant mice models. Results: Expression patterns of the joint marker Gdf5, tendon and ligament marker Scleraxis, early muscle marker MyoD1, and blood vessel marker Cadherin5 (Cdh5) are presented during the most active phases of embryonic mouse limb patterning. Anti‐neurofilament staining of developing nerves in the fore‐ and hindlimbs and cartilage formation and progression also are described. Conclusions: This study demonstrates and describes a range of key morphological markers and methods that together can be used to assess normal and abnormal limb development. Developmental Dynamics 247:1217–1226, 2018. © 2018 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists

Expression patterns of molecular markers throughout both fore‐ and hindlimb development ‐ which can be used to assess normal and abnormal development.

Detailled description of innervation during fore‐ and hindlimb development confirming innervation first seen after limb patterning events have begun.

Description of cartilage development and progression indicates alizarin red staining not seen until E15.5 in both fore‐ and hindlimbs.

Hindlimb lags behind forelimb molecularly and morphologically until E14.5.

Detailled description of methods used to study fore‐ and hindlimb development.

Ectopic BAT mUCP-1 overexpression in SKM by delivering a BMP7/PRDM16/PGC-1a gene cocktail or single PRMD16 using non-viral UTMD gene therapy

Here we present our progress in inducing an ectopic brown adipose tissue (BAT) phenotype in skeletal muscle (SKM) as a potential gene therapy for obesity and its comorbidities. We used ultrasound-targeted microbubble destruction (UTMD), a novel targeted, non-viral approach to gene therapy, to deliver genes in the BAT differentiation pathway into rodent SKM to engineer a thermogenic BAT phenotype with ectopic mUCP-1 overexpression. In parallel, we performed a second protocol using wild-type Ucp-1-null knockout mice to test whether the effects of the gene therapy are UCP-1 dependent. Our main findings were a robust cellular presence of mUCP-1 immunostaining (IHC), significantly higher expression levels of mUCP-1 measured by qRT-PCR, and highest temperature elevation measured by infrared thermography in the treated thigh, achieved in rats after delivering the UTMD-PRDM16/PGC-1a/BMP7/hyPB gene cocktail. Interestingly, the weight loss obtained in the treated rats with the triple gene delivery, never recovered the levels observed in the controls in spite of food intake recovery. Our results establish the feasibility of minimally invasive UTMD gene-based therapy administration in SKM, to induce overexpression of ectopic mUCP-1 after delivery of the thermogenic BAT gene program, and describe systemic effects of this intervention on food intake, weight loss, and thermogenesis.

The requirement of Mettl3-promoted MyoD mRNA maintenance in proliferative myoblasts for skeletal muscle differentiation

Myogenic progenitor/stem cells retain their skeletal muscle differentiation potential by maintaining myogenic transcription factors such as MyoD. However, the mechanism of how MyoD expression is maintained in proliferative progenitor cells has not been elucidated. Here, we found that MyoD expression was reduced at the mRNA level by cell cycle arrest in S and G2 phases, which in turn led to the absence of skeletal muscle differentiation. The reduction of MyoD mRNA was correlated with the reduced expression of factors regulating RNA metabolism, including methyltransferase like 3 (Mettl3), which induces N⁶-methyladenosine (m⁶A) modifications of RNA. Knockdown of Mettl3 revealed that MyoD RNA was specifically downregulated and that this was caused by a decrease in processed, but not unprocessed, mRNA. Potential m⁶A modification sites were profiled by m⁶A sequencing and identified within the 5' untranslated region (UTR) of MyoD mRNA. Deletion of the 5' UTR revealed that it has a role in MyoD mRNA processing. These data showed that Mettl3 is required for MyoD mRNA expression in proliferative myoblasts.

Reporter-Based Isolation of Developmental Myogenic Progenitors

The formation and activity of mammalian tissues entail finely regulated processes, involving the concerted organization and interaction of multiple cell types. In recent years the prospective isolation of distinct progenitor and stem cell populations has become a powerful tool in the hands of developmental biologists and has rendered the investigation of their intrinsic properties possible. In this protocol, we describe how to purify progenitors with different lineage history and degree of differentiation from embryonic and fetal skeletal muscle by fluorescence-activated cell sorting (FACS). The approach takes advantage of a panel of murine strains expressing fluorescent reporter genes specifically in the myogenic progenitors. We provide a detailed description of the dissection procedures and of the enzymatic dissociation required to maximize the yield of mononucleated cells for subsequent FACS-based purification. The procedure takes ~6–7 h to complete and allows for the isolation and the subsequent molecular and phenotypic characterization of developmental myogenic progenitors.

Loss of MyoD and Myf5 in Skeletal Muscle Stem Cells Results in Altered Myogenic Programming and Failed Regeneration

MyoD and Myf5 are fundamental regulators of skeletal muscle lineage determination in the embryo, and their expression is induced in satellite cells following muscle injury. MyoD and Myf5 are also expressed by satellite cell precursors developmentally, although the relative contribution of historical and injury-induced expression to satellite cell function is unknown. We show that satellite cells lacking both MyoD and Myf5 (double knockout [dKO]) are maintained with aging in uninjured muscle. However, injured muscle fails to regenerate and dKO satellite cell progeny accumulate in damaged muscle but do not undergo muscle differentiation. dKO satellite cell progeny continue to express markers of myoblast identity, although their myogenic programming is labile, as demonstrated by dramatic morphological changes and increased propensity for non-myogenic differentiation. These data demonstrate an absolute requirement for either MyoD or Myf5 in muscle regeneration and indicate that their expression after injury stabilizes myogenic identity and confers the capacity for muscle differentiation.

•

MyoD or Myf5 expression in satellite cells is essential for muscle regeneration

•

Satellite cells lacking both regulatory genes exhibit labile myogenic programming

•

A single functional allele of either MyoD or Myf5 can support muscle regeneration

•

Satellite cells lacking both MyoD and Myf5 are maintained with aging

Activin-dependent signaling in fibro/adipogenic progenitors causes fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal-dominant disorder characterized by progressive and profoundly disabling heterotopic ossification (HO). Here we show that fibro/adipogenic progenitors (FAPs) are a major cell-of-origin of HO in an accurate genetic mouse model of FOP (Acvr1^tnR206H). Targeted expression of the disease-causing type I bone morphogenetic protein (BMP) receptor, ACVR1(R206H), to FAPs recapitulates the full spectrum of HO observed in FOP patients. ACVR1(R206H)-expressing FAPs, but not wild-type FAPs, activate osteogenic signaling in response to activin ligands. Conditional loss of the wild-type Acvr1 allele dramatically exacerbates FAP-directed HO, suggesting that mutant and wild-type ACVR1 receptor complexes compete for activin ligands or type II BMP receptor binding partners. Finally, systemic inhibition of activin A completely blocks HO and restores wild-type-like behavior to transplanted Acvr1^R206H/+ FAPs. Understanding the cells that drive HO may facilitate the development of cell-specific therapeutic approaches to inhibit catastrophic bone formation in FOP.

Fibrodysplasia ossificans progressiva is a severe disorder characterized by heterotopic ossification, and is caused by mutations in ACVR1. Here, the authors show that expression of mutant ACVR1 in fibro/adipogenic progenitors recapitulates disease progression, and that this can be halted by systemic inhibition of activin A in mice.

Function of the myogenic regulatory factors Myf5, MyoD, Myogenin and MRF4 in skeletal muscle, satellite cells and regenerative myogenesis

Discovery of the myogenic regulatory factor family of transcription factors MYF5, MYOD, Myogenin and MRF4 was a seminal step in understanding specification of the skeletal muscle lineage and control of myogenic differentiation during development. These factors are also involved in specification of the muscle satellite cell lineage, which becomes the resident stem cell compartment inadult skeletal muscle. While MYF5, MYOD, Myogenin and MRF4 have subtle roles in mature muscle, they again play a crucial role in directing satellite cell function to regenerate skeletal muscle: linking the genetic control of developmental and regenerative myogenesis. Here, I review the role of the myogenic regulatory factors in developing and mature skeletal muscle, satellite cell specification and muscle regeneration.

Staufen1 inhibits MyoD translation to actively maintain muscle stem cell quiescence

Tissue regeneration depends on the timely activation of adult stem cells. In skeletal muscle, the adult stem cells maintain a quiescent state and proliferate upon injury. We show that muscle stem cells (MuSCs) use direct translational repression to maintain the quiescent state. High-resolution single-molecule and single-cell analyses demonstrate that quiescent MuSCs express high levels of Myogenic Differentiation 1 (MyoD) transcript in vivo, whereas MyoD protein is absent. RNA pulldowns and costainings show that MyoD mRNA interacts with Staufen1, a potent regulator of mRNA localization, translation, and stability. Staufen1 prevents MyoD translation through its interaction with the MyoD 3'-UTR. MuSCs from Staufen1 heterozygous (Staufen1^+/-) mice have increased MyoD protein expression, exit quiescence, and begin proliferating. Conversely, blocking MyoD translation maintains the quiescent phenotype. Collectively, our data show that MuSCs express MyoD mRNA and actively repress its translation to remain quiescent yet primed for activation.

Relationships among muscle fiber type composition, fiber diameter and MRF gene expression in different skeletal muscles of naturally grazing Wuzhumuqin sheep during postnatal development

The aim of this study was to determine the relationships among muscle fiber‐type composition, fiber diameter, and myogenic regulatory factor (MRF) gene expression in different skeletal muscles during development in naturally grazing Wuzhumuqin sheep. Three major muscles (i.e. the Longissimus dorsi (LD), Biceps femoris (BF) and Triceps brachii (TB)) were obtained from 20 Wuzhumuqin sheep and 20 castrated rams at each of the following ages: 1, 3, 6, 9, 12 and 18 months. Muscle fiber‐type composition and fiber diameter were measured using histochemistry and morphological analysis, and MRF gene expression levels were determined using real‐time PCR. In the LD muscle, changes in the proportion of each of different types of fiber (I, IIA and IIB) were relatively small. In the BF muscle, a higher proportion of type I and a 6.19‐fold lower proportion of type IIA fibers were observed (P < 0.05). In addition, the compositions of type I and IIA fibers continuously changed in the TB muscle (P < 0.05). Moreover, muscle diameter gradually increased throughout development (P < 0.05). Almost no significant difference was found in MRF gene expression patterns, which appeared to be relatively stable. These results suggest that changes in fiber‐type composition and increases in fiber size may be mutually interacting processes during muscle development.

An Examination of the Role of Transcriptional and Posttranscriptional Regulation in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is an aggressive family of soft tissue tumors that most commonly manifests in children. RMS variants express several skeletal muscle markers, suggesting myogenic stem or progenitor cell origin of RMS. In this review, the roles of both recently identified and well-established microRNAs in RMS are discussed and summarized in a succinct, tabulated format. Additionally, the subtypes of RMS are reviewed along with the involvement of basic helix-loop-helix (bHLH) proteins, Pax proteins, and microRNAs in normal and pathologic myogenesis. Finally, the current and potential future treatment options for RMS are outlined.

The Potential of Combination Therapeutics for More Complete Repair of Volumetric Muscle Loss Injuries: The Role of Exogenous Growth Factors and/or Progenitor Cells in Implantable Skeletal Muscle Tissue Engineering Technologies

Despite the robust regenerative capacity of skeletal muscle, there are a variety of congenital and acquired conditions in which the volume of skeletal muscle loss results in major permanent functional and cosmetic deficits. These latter injuries are referred to as volumetric muscle loss (VML) injuries or VML-like conditions, and they are characterized by the simultaneous absence of multiple tissue components (i.e., nerves, vessels, muscles, satellite cells, and matrix). There are currently no effective treatment options. Regenerative medicine/tissue engineering technologies hold great potential for repair of these otherwise irrecoverable VML injuries. In this regard, three-dimensional scaffolds have been used to deliver sustained amounts of growth factors into a variety of injury models, to modulate host cell recruitment and extracellular matrix remodeling. However, this is a nascent field of research, and more complete functional improvements require more precise control of the spatiotemporal distribution of critical growth factors over a physiologically relevant range. This is especially true for VML injuries where incorporation of a cellular component into the scaffolds might provide not only a source of new tissue formation but also additional signals for host cell migration, recruitment, and survival. To this end, we review the major features of muscle repair and regeneration for largely recoverable injuries, and then discuss recent cell- and/or growth factor-based approaches to repair the more profound and irreversible VML and VML-like injuries. The underlying supposition is that more rationale incorporation of exogenous growth factors and/or cellular components will be required to optimize the regenerative capacity of implantable therapeutics for VML repair.

Convective exosome-tracing microfluidics for analysis of cell-non-autonomous neurogenesis

The effective role of exosome delivering neurogenic microRNA (miRNA) enables to induce efficient differentiation process during neurogenesis. The microfludic system capable of visualizing the exosomal behavior such as secretion, migration, and uptake of individual exosomes can be used as a robust technique to understand the exosome-mediated change of cellular behavior. Here, we developed the exosome-tracing microfluidic system to visualize exosomal transport carrying the neurogenic miRNA from leading to neighboring cells, and found a new mode of exosome-mediated cell-non-autonomous neurogenesis. The miR-193a facilitated neurogenesis in F11 cells by blocking proliferation-related target genes. In addition to time-lapse live-cell imaging using microfluidics visualized the convective transport of exosomes from differentiated to undifferentiated cells. Individual exosomes containing miR-193a from differentiated donor cells were taken up by undifferentiated cells to lead them to neurogenesis. Induction of anti-miR-193a was sufficient to block neurogenesis in F11 cells. Inhibition of the exosomal production by manumycin-A and treatment of anti-miR-193a in the differentiated donor cells failed to induce neurogenesis in undifferentiated recipient cells. These findings indicate that exosomes of neural progenitors and neurogenic miRNA within these exosomes propagate cell-non-autonomous differentiation to neighboring progenitors, to delineate the roles of exosome mediating neurogenesis of population of homologous neural progenitor cells.

The lysine methyltransferase Ehmt2/G9a is dispensable for skeletal muscle development and regeneration

Background

Euchromatic histone-lysine N-methyltransferase 2 (G9a/Ehmt2) is the main enzyme responsible for the apposition of H3K9 di-methylation on histones. Due to its dual role as an epigenetic regulator and in the regulation of non-histone proteins through direct methylation, G9a has been implicated in a number of biological processes relevant to cell fate control. Recent reports employing in vitro cell lines indicate that Ehmt2 methylates MyoD to repress its transcriptional activity and therefore its ability to induce differentiation of activated myogenic cells.

Methods

To further investigate the importance of G9a in modulating myogenic regeneration in vivo, we crossed Ehmt2^floxed mice to animals expressing Cre recombinase from the Myod locus, resulting in efficient knockout in the entire skeletal muscle lineage (Ehmt2^ΔmyoD).

Results

Surprisingly, despite a dramatic drop in the global levels of H3K9me2, knockout animals did not show any developmental phenotype in muscle size and appearance. Consistent with this finding, purified Ehmt2^ΔmyoD satellite cells had rates of activation and proliferation similar to wild-type controls. When induced to differentiate in vitro, Ehmt2 knockout cells differentiated with kinetics similar to those of control cells and demonstrated normal capacity to form myotubes. After acute muscle injury, knockout mice regenerated as efficiently as wildtype. To exclude possible compensatory mechanisms elicited by the loss of G9a during development, we restricted the knockout within adult satellite cells by crossing Ehmt2^floxed mice to Pax7^CreERT2 and also found normal muscle regeneration capacity.

Conclusions

Thus, Ehmt2 and H3K9me2 do not play significant roles in skeletal muscle development and regeneration in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0093-7) contains supplementary material, which is available to authorized users.

Satellite Cell Heterogeneity in Skeletal Muscle Homeostasis

The cellular turnover required for skeletal muscle maintenance and repair is mediated by resident stem cells, also termed satellite cells. Satellite cells normally reside in a quiescent state, intermittently entering the cell cycle to fuse with neighboring myofibers and replenish the stem cell pool. However, the mechanisms by which satellite cells maintain the precise balance between self-renewal and differentiation necessary for long-term homeostasis remain unclear. Recent work has supported a previously unappreciated heterogeneity in the satellite cell compartment that may underlie the observed variability in cell fate and function. In this review, we examine the work supporting this notion as well as the potential governing principles, developmental origins, and principal determinants of satellite cell heterogeneity.

Chitooligomer-Immobilized Biointerfaces with Micropatterned Geometries for Unidirectional Alignment of Myoblast Cells

Skeletal muscle possesses a robust capacity to regenerate functional architectures with a unidirectional orientation. In this study, we successfully arranged skeletal myoblast (C2C12) cells along micropatterned gold strips on which chitohexaose was deposited via a vectorial chain immobilization approach. Hexa-N-acetyl-d-glucosamine (GlcNAc6) was site-selectively modified at its reducing end with thiosemicarbazide, then immobilized on a gold substrate in striped micropatterns via S–Au chemisorption. Gold micropatterns ranged from 100 to 1000 µm in width. Effects of patterning geometries on C2C12 cell alignment, morphology, and gene expression were investigated. Unidirectional alignment of C2C12 cells having GlcNAc6 receptors was clearly observed along the micropatterns. Decreasing striped pattern width increased cell attachment and proliferation, suggesting that the fixed GlcNAc6 and micropatterns impacted cell function. Possibly, interactions between nonreducing end groups of fixed GlcNAc6 and cell surface receptors initiated cellular alignment. Our technique for mimicking native tissue organization should advance applications in tissue engineering.

Deficiency of the myogenic factor MyoD causes a perinatally lethal fetal akinesia

Background

Lethal fetal akinesia deformation sequence (FADS) describes a clinically and genetically heterogeneous phenotype that includes fetal akinesia, intrauterine growth retardation, arthrogryposis and developmental anomalies. Affected babies die as a result of pulmonary hypoplasia. We aimed to identify the underlying genetic cause of this disorder in a family in which there were three affected individuals from two sibships.

Methods

Autosomal-recessive inheritance was suggested by a family history of consanguinity and by recurrence of the phenotype between the two sibships. We performed exome sequencing of the affected individuals and their unaffected mother, followed by autozygosity mapping and variant filtering to identify the causative gene.

Results

Five autozygous regions were identified, spanning 31.7 Mb of genomic sequence and including 211 genes. Using standard variant filtering criteria, we excluded all variants as being the likely pathogenic cause, apart from a single novel nonsense mutation, c.188C>A p.(Ser63*) (NM_002478.4), in MYOD1. This gene encodes an extensively studied transcription factor involved in muscle development, which has nonetheless not hitherto been associated with a hereditary human disease phenotype.

Conclusions

We provide the first description of a human phenotype that appears to result from MYOD1 mutation. The presentation with FADS is consistent with a large body of data demonstrating that in the mouse, MyoD is a major controller of precursor cell commitment to the myogenic differentiation programme.

Contributions of muscle-resident progenitor cells to homeostasis and disease

Adult skeletal muscle maintains a homeostatic state with modest levels of cellular turnover, unlike the skin or blood. However, the muscle is highly sensitive to tissue injury, which unleashes a cascade of regenerative and inflammatory processes. Muscle regeneration involves cross-talk between numerous cytokine signaling axes, and the coordinated activity of multiple muscle-resident and circulating progenitor populations. Satellite cells, closely associated with myofibers, are established as the canonical muscle stem cell, with self-renewal and myofiber-regenerating capacity. However, a heterogeneous group of mesenchymal progenitor cells residing within the muscle interstitium are also highly responsive to muscle injury and exhibit varying degrees of regenerative potential. These cells interact with satellite cells via direct and indirect mechanisms to regulate regeneration or repair. We describe the known phylogenetic and functional relationships of the multiple progenitor populations residing within skeletal muscle, their putative roles in the coordination of injury repair, and their possible contributions to health and disease.

Positive effect of IGF-1 injection on gastrocnemius of rat during distraction osteogenesis

Distraction osteogenesis (DO) is used to form new bone between bone segments to lengthen the callus. Skeletal muscles frequently fail to adapt to distraction, which causes complications. Insulin-like growth factor-1 (IGF-1) has been implicated as a central regulator of muscle repair. We hypothesized that IGF-1 injection could reduce muscle complications in DO. A total of 102 Sprague-Dawley rats received DO or did not were randomly assigned into saline, IGF-1 and normal groups. On the day before the distraction, the rats in the IGF-1 group were injected with IGF-1. The gastrocnemius muscles of the rats were harvested at the 0, 1st, 4th, 7th, and 10th days of distraction. The weight of the muscles, cross-sectional area (CSA) of the muscle fibers, collagen volume fraction (CVF), maximum limit load (MLL), maximum contraction forces, and gene expression of Akt, MyoD, myogenin, myostatin, and collagen I were analyzed. The results indicated that IGF-1 injection had increased the weights, CSA of the muscle fibers, MLL and force generation of the gastrocnemius. Also, Akt, MyoD, and myogenin were upregulated, and myostatin was downregulated in the IGF-1 group. Injection of IGF-1 could attenuate the gastrocnemius atrophy, prevent fibrosis, increase MLL, and regulate the related mRNA.

Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

STATEMENT OF SIGNIFICANCE

Volumetric muscle loss (VML) injuries result from traumatic incidents such as those presented from combat missions, where soft-tissue extremity injuries are represented in one of four cases. These injuries remove or destroy large amounts of skeletal muscle including the basement membrane and connective tissue, removing the structural, mechanical, and biochemical cues that usually direct its repair. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. In this review, we examine current strategies for the development of scaffold materials designed for skeletal muscle regeneration, highlighting advances and limitations associated with these methodologies. Finally, we identify future approaches to enhance skeletal muscle regeneration.

Signaling Pathways Related to Protein Synthesis and Amino Acid Concentration in Pig Skeletal Muscles Depend on the Dietary Protein Level, Genotype and Developmental Stages

Muscle growth is regulated by the homeostatic balance of the biosynthesis and degradation of muscle proteins. To elucidate the molecular interactions among diet, pig genotype, and physiological stage, we examined the effect of dietary protein concentration, pig genotype, and physiological stages on amino acid (AA) pools, protein deposition, and related signaling pathways in different types of skeletal muscles. The study used 48 Landrace pigs and 48 pure-bred Bama mini-pigs assigned to each of 2 dietary treatments: lower/GB (Chinese conventional diet)- or higher/NRC (National Research Council)-protein diet. Diets were fed from 5 weeks of age to respective market weights of each genotype. Samples of biceps femoris muscle (BFM, type I) and longissimus dorsi muscle (LDM, type II) were collected at nursery, growing, and finishing phases according to the physiological stage of each genotype, to determine the AA concentrations, mRNA levels for growth-related genes in muscles, and protein abundances of mechanistic target of rapamycin (mTOR) signaling pathway. Our data showed that the concentrations of most AAs in LDM and BFM of pigs increased (P<0.05) gradually with increasing age. Bama mini-pigs had generally higher (P<0.05) muscle concentrations of flavor-related AA, including Met, Phe, Tyr, Pro, and Ser, compared with Landrace pigs. The mRNA levels for myogenic determining factor, myogenin, myocyte-specific enhancer binding factor 2 A, and myostatin of Bama mini-pigs were higher (P<0.05) than those of Landrace pigs, while total and phosphorylated protein levels for protein kinase B, mTOR, and p70 ribosomal protein S6 kinases (p70S6K), and ratios of p-mTOR/mTOR, p-AKT/AKT, and p-p70S6K/p70S6K were lower (P<0.05). There was a significant pig genotype-dependent effect of dietary protein on the levels for mTOR and p70S6K. When compared with the higher protein-NRC diet, the lower protein-GB diet increased (P<0.05) the levels for mTOR and p70S6K in Bama mini-pigs, but repressed (P<0.05) the level for p70S6K in Landrace pigs. The higher protein-NRC diet increased ratio of p-mTOR/mTOR in Landrace pigs. These findings indicated that the dynamic consequences of AA profile and protein deposition in muscle tissues are the concerted effort of distinctive genotype, nutrient status, age, and muscle type. Our results provide valuable information for animal feeding strategy.

Lkb1 is indispensable for skeletal muscle development, regeneration, and satellite cell homeostasis

Serine/threonine kinase 11, commonly known as liver kinase b1 (Lkb1), is a tumor suppressor that regulates cellular energy metabolism and stem cell function. Satellite cells are skeletal muscle resident stem cells that maintain postnatal muscle growth and repair. Here, we used MyoD(Cre)/Lkb1(flox/flox) mice (called MyoD-Lkb1) to delete Lkb1 in embryonic myogenic progenitors and their descendant satellite cells and myofibers. The MyoD-Lkb1 mice exhibit a severe myopathy characterized by central nucleated myofibers, reduced mobility, growth retardation, and premature death. Although tamoxifen-induced postnatal deletion of Lkb1 in satellite cells using Pax7(CreER) mice bypasses the developmental defects and early death, Lkb1 null satellite cells lose their regenerative capacity cell-autonomously. Strikingly, Lkb1 null satellite cells fail to maintain quiescence in noninjured resting muscles and exhibit accelerated proliferation but reduced differentiation kinetics. At the molecular level, Lkb1 limits satellite cell proliferation through the canonical AMP-activated protein kinase/mammalian target of rapamycin pathway, but facilitates differentiation through phosphorylation of GSK-3β, a key component of the WNT signaling pathway. Together, these results establish a central role of Lkb1 in muscle stem cell homeostasis, muscle development, and regeneration.

Expression of myogenic regulatory factors in chicken embryos during somite and limb development

The expression of the myogenic regulatory factors (MRFs), Myf5, MyoD, myogenin (Mgn) and MRF4 have been analysed during the development of chicken embryo somites and limbs. In somites, Myf5 is expressed first in somites and paraxial mesoderm at HH stage 9 followed by MyoD at HH stage 12, and Mgn and MRF4 at HH stage 14. In older somites, Myf5 and MyoD are also expressed in the ventrally extending myotome prior to Mgn and MRF4 expression. In limb muscles a similar temporal sequence is observed with Myf5 expression detected first in forelimbs at HH stage 22, MyoD at HH stage 23, Mgn at HH stage 24 and MRF4 at HH stage 30. This report describes the precise time of onset of expression of each MRF in somites and limbs during chicken embryo development, and provides a detailed comparative timeline of MRF expression in different embryonic muscle groups.

The emergence of Pax7-expressing muscle stem cells during vertebrate head muscle development

Pax7 expressing muscle stem cells accompany all skeletal muscles in the body and in healthy individuals, efficiently repair muscle after injury. Currently, the in vitro manipulation and culture of these cells is still in its infancy, yet muscle stem cells may be the most promising route toward the therapy of muscle diseases such as muscular dystrophies. It is often overlooked that muscular dystrophies affect head and body skeletal muscle differently. Moreover, these muscles develop differently. Specifically, head muscle and its stem cells develop from the non-somitic head mesoderm which also has cardiac competence. To which extent head muscle stem cells retain properties of the early head mesoderm and might even be able to switch between a skeletal muscle and cardiac fate is not known. This is due to the fact that the timing and mechanisms underlying head muscle stem cell development are still obscure. Consequently, it is not clear at which time point one should compare the properties of head mesodermal cells and head muscle stem cells. To shed light on this, we traced the emergence of head muscle stem cells in the key vertebrate models for myogenesis, chicken, mouse, frog and zebrafish, using Pax7 as key marker. Our study reveals a common theme of head muscle stem cell development that is quite different from the trunk. Unlike trunk muscle stem cells, head muscle stem cells do not have a previous history of Pax7 expression, instead Pax7 expression emerges de-novo. The cells develop late, and well after the head mesoderm has committed to myogenesis. We propose that this unique mechanism of muscle stem cell development is a legacy of the evolutionary history of the chordate head mesoderm.

Different requirements for Wnt signaling in tongue myogenic subpopulations

The tongue is a muscular organ that is essential in vertebrates for important functions, such as food intake and communication. Little is known about regulation of myogenic progenitors during tongue development when compared with the limb or trunk region. In this study, we investigated the relationship between different myogenic subpopulations and the function of canonical Wnt signaling in regulating these subpopulations. We found that Myf5- and MyoD-expressing myogenic subpopulations exist during embryonic tongue myogenesis. In the Myf5-expressing myogenic progenitors, there is a cell-autonomous requirement for canonical Wnt signaling for cell migration and differentiation. In contrast, the MyoD-expressing subpopulation does not require canonical Wnt signaling during tongue myogenesis. Taken together, our results demonstrate that canonical Wnt signaling differentially regulates the Myf5- and MyoD-expressing subpopulations during tongue myogenesis.

Interactions between FGF18 and retinoic acid regulate differentiation of chick embryo limb myoblasts

During limb development Pax3 positive myoblasts delaminate from the hypaxial dermomyotome of limb level somites and migrate into the limb bud where they form the dorsal and ventral muscle masses. Only then do they begin to differentiate and express markers of myogenic commitment and determination such as Myf5 and MyoD. However the signals regulating this process remain poorly characterised. We show that FGF18, which is expressed in the distal mesenchyme of the limb bud, induces premature expression of both Myf5 and MyoD and that blocking FGF signalling also inhibits endogenous MyoD expression. This expression is mediated by ERK MAP kinase but not PI3K signalling. We also show that retinoic acid (RA) can inhibit the myogenic activity of FGF18 and that blocking RA signalling allows premature induction of MyoD by FGF18 at HH19. We propose a model where interactions between FGF18 in the distal limb and retinoic acid in the proximal limb regulate the timing of myogenic gene expression during limb bud development.

Expression pattern of mRNA A and mRNA B of alpha sarcoglycan gene during mouse embryonic development and regulation of their expression by myogenic and cardiogenic transcription factors

BACKGROUND

Type 2D limb-girdle muscular dystrophy (LGM2D) is a progressive disorder caused by mutations in the alpha sarcoglycan (α-SG) gene. In mice, the α-SG gene contains two promoters that regulate the expression of two different mRNAs (A and B). However, their gene expression pattern during embryonic development has not been explored and their regulation by myogenic and cardiogenic transcription factors has been only partially studied.

RESULTS

During embryonic development, mRNA A and B of α-SG gene were initially detected in hypaxial muscles, heart, stomach, tongue, and mesenchymal cells, which surround the dorsal region of the somites. Moreover, mRNA B was exclusively expressed in the floor plate and notochord and in the interdigits of limbs. In vitro, MyoD and myogenin positively regulated the transcription of mRNA B during skeletal myogenesis, whereas mRNA A was activated only for MyoD in differentiated skeletal muscle. In addition, Gata-4 together with Mef2c may regulate the expression of mRNA B in heart development, whereas Nkx2.5 and myocardin may activate expression of mRNA A in the differentiated cardiomyocyte.

CONCLUSIONS

The differential expression of α-SG mRNAs during mouse embryonic development may be a consequence of the differential regulation of both promoters by myogenic and cardiogenic 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.

Roles of nonmyogenic mesenchymal progenitors in pathogenesis and regeneration of skeletal muscle

Adult skeletal muscle possesses a remarkable regenerative ability that is dependent on satellite cells. However, skeletal muscle is replaced by fatty and fibrous connective tissue in several pathological conditions. Fatty and fibrous connective tissue becomes a major cause of muscle weakness and leads to further impairment of muscle function. Because the occurrence of fatty and fibrous connective tissue is usually associated with severe destruction of muscle, the idea that dysregulation of the fate switch in satellite cells may underlie this pathological change has emerged. However, recent studies identified nonmyogenic mesenchymal progenitors in skeletal muscle and revealed that fatty and fibrous connective tissue originates from these progenitors. Later, these progenitors were also demonstrated to be the major contributor to heterotopic ossification in skeletal muscle. Because nonmyogenic mesenchymal progenitors represent a distinct cell population from satellite cells, targeting these progenitors could be an ideal therapeutic strategy that specifically prevents pathological changes of skeletal muscle, while preserving satellite cell-dependent regeneration. In addition to their roles in pathogenesis of skeletal muscle, nonmyogenic mesenchymal progenitors may play a vital role in muscle regeneration by regulating satellite cell behavior. Conversely, muscle cells appear to regulate behavior of nonmyogenic mesenchymal progenitors. Thus, these cells regulate each other reciprocally and a proper balance between them is a key determinant of muscle integrity. Furthermore, nonmyogenic mesenchymal progenitors have been shown to maintain muscle mass in a steady homeostatic condition. Understanding the nature of nonmyogenic mesenchymal progenitors will provide valuable insight into the pathophysiology of skeletal muscle. In this review, we focus on nonmyogenic mesenchymal progenitors and discuss their roles in muscle pathogenesis, regeneration, and homeostasis.