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

SIRT1-dependent mechanisms and effects of resveratrol for amelioration of muscle wasting in NASH mice

Background

In non-alcoholic steatohepatitis (NASH), muscle wasting was an aggravating factor for the progression of hepatic steatosis. This study explores the potential benefits of chronic treatment with resveratrol, a strong activator of SIRT1 on the muscle wasting of NASH mice.

Methods

In vivo and in vitro study, we evaluate the SIRT1-dependent mechanisms and effects of resveratrol administration for 6 weeks with high-fat-methionine and choline deficient diet-induced NASH mice and palmitate-pretreated C2C12 myoblast cells.

Results

Resveratrol treatment improved grip strength and muscle mass of limbs, increased running distance and time on exercise wheels in NASH mice. There is a negative correlation between muscular SIRT1 activity and 3-nitrotyrosine levels of NASH and NASH-resv mice. The SIRT1-dependent effect of muscle wasting was associated with the suppression of oxidative stress, upregulation of antioxidants, inhibition of protein degradation, activation of autophagy, suppression of apoptotic activity, upregulation of lipolytic genes and the reduction of fatty infiltration in limb muscles of NASH mice. In vitro, resveratrol alleviated palmitate acid-induced oxidative stress, lipid deposition, autophagy dysfunction, apoptotic signals, and subsequently reduced fusion index and myotube formation of C2C12 cells. The beneficial effects of resveratrol were abolished by EX527.

Conclusions

Our study suggests that chronic resveratrol treatment is a potential strategy for amelioration of hepatic steatosis and muscle wasting in NASH mouse model.

IL-4 and SDF-1 Increase Adipose Tissue-Derived Stromal Cell Ability to Improve Rat Skeletal Muscle Regeneration

Skeletal muscle regeneration depends on the satellite cells, which, in response to injury, activate, proliferate, and reconstruct damaged tissue. However, under certain conditions, such as large injuries or myopathies, these cells might not sufficiently support repair. Thus, other cell populations, among them adipose tissue-derived stromal cells (ADSCs), are tested as a tool to improve regeneration. Importantly, the pro-regenerative action of such cells could be improved by various factors. In the current study, we tested whether IL-4 and SDF-1 could improve the ability of ADSCs to support the regeneration of rat skeletal muscles. We compared their effect at properly regenerating fast-twitch EDL and poorly regenerating slow-twitch soleus. To this end, ADSCs subjected to IL-4 and SDF-1 were analyzed in vitro and also in vivo after their transplantation into injured muscles. We tested their proliferation rate, migration, expression of stem cell markers and myogenic factors, their ability to fuse with myoblasts, as well as their impact on the mass, structure and function of regenerating muscles. As a result, we showed that cytokine-pretreated ADSCs had a beneficial effect in the regeneration process. Their presence resulted in improved muscle structure and function, as well as decreased fibrosis development and a modulated immune response.

Methods for Accurate Assessment of Myofiber Maturity During Skeletal Muscle Regeneration

Adult skeletal muscle has a remarkable ability to regenerate. Regeneration of mature muscle fibers is dependent on muscle stem cells called satellite cells. Although they are normally in a quiescent state, satellite cells are rapidly activated after injury, and subsequently proliferate and differentiate to make new muscle fibers. Myogenesis is a highly orchestrated biological process and has been extensively studied, and therefore many parameters that can precisely evaluate regenerating events have been established. However, in some cases, it is necessary to evaluate the completion of regeneration rather than ongoing regeneration. In this study, we establish methods for assessing the myofiber maturation during muscle regeneration. By carefully comparing expression patterns of several muscle regeneration-related genes, we found that expression of Myozenin (Myoz1 and Myoz3), Troponin I (Tnni2), and Dystrophin (Dmd) is gradually increased as muscle regeneration proceeds. In contrast, commonly used regeneration markers such as Myh3 and Myh8 are transiently upregulated after muscle injury but their expression decreases as regeneration progresses. Intriguingly, upregulation of Myoz1, Myoz3 and Tnni2 cannot be achieved in cultured myotubes, indicating that these markers are excellent indicators to assess myofiber maturity. We also show that analyzing re-expression of Myoz1 and dystrophin in individual fiber during regeneration enables accurate assessment of myofiber maturity at the single-myofiber level. Together, our study provides valuable methods that are useful in evaluating muscle regeneration and the efficacy of therapeutic strategies for muscle diseases.

Adiponectin and Its Mimics on Skeletal Muscle: Insulin Sensitizers, Fat Burners, Exercise Mimickers, Muscling Pills … or Everything Together?

Adiponectin (ApN) is a hormone abundantly secreted by adipocytes and it is known to be tightly linked to the metabolic syndrome. It promotes insulin-sensitizing, fat-burning, and anti-atherosclerotic actions, thereby effectively counteracting several metabolic disorders, including type 2 diabetes, obesity, and cardiovascular diseases. ApN is also known today to possess powerful anti-inflammatory/oxidative and pro-myogenic effects on skeletal muscles exposed to acute or chronic inflammation and injury, mainly through AdipoR1 (ApN specific muscle receptor) and AMP-activated protein kinase (AMPK) pathway, but also via T-cadherin. In this review, we will report all the beneficial and protective properties that ApN can exert, specifically on the skeletal muscle as a target tissue. We will highlight its effects and mechanisms of action, first in healthy skeletal muscle including exercised muscle, and second in diseased muscle from a variety of pathological conditions. In the end, we will go over some of AdipoRs agonists that can be easily produced and administered, and which can greatly mimic ApN. These interesting and newly identified molecules could pave the way towards future therapeutic approaches to potentially prevent or combat not only skeletal muscle disorders but also a plethora of other diseases with sterile inflammation or metabolic dysfunction.

Generation of craniofacial myogenic progenitor cells from human induced pluripotent stem cells for skeletal muscle tissue regeneration

Craniofacial skeletal muscle is composed of approximately 60 muscles, which have critical functions including food uptake, eye movements and facial expressions. Although craniofacial muscles have significantly different embryonic origin, most current skeletal muscle differentiation protocols using human induced pluripotent stem cells (iPSCs) are based on somite-derived limb and trunk muscle developmental pathways. Since the lack of a protocol for craniofacial muscles is a significant gap in the iPSC-derived muscle field, we have developed an optimized protocol to generate craniofacial myogenic precursor cells (cMPCs) from human iPSCs by mimicking key signaling pathways during craniofacial embryonic myogenesis. At each different stage, human iPSC-derived cMPCs mirror the transcription factor expression profiles seen in their counterparts during embryo development. After the bi-potential cranial pharyngeal mesoderm is established, cells are committed to cranial skeletal muscle lineages with inhibition of cardiac lineages and are purified by flow cytometry. Furthermore, identities of Ipsc-derived cMPCs are verified with human primary myoblasts from craniofacial muscles using RNA sequencing. These data suggest that our new method could provide not only in vitro research tools to study muscle specificity of muscular dystrophy but also abundant and reliable cellular resources for tissue engineering to support craniofacial reconstruction surgery.

AdipoRon, a new therapeutic prospect for Duchenne muscular dystrophy

BACKGROUND

Adiponectin (ApN) is a hormone known to exhibit insulin-sensitizing, fat-burning, and anti-inflammatory properties in several tissues, including the skeletal muscle. Duchenne muscular dystrophy (DMD) is a devastating disease characterized by dystrophin deficiency with subsequent chronic inflammation, myofiber necrosis, and impaired regeneration. Previously, we showed that transgenic up-regulation of ApN could significantly attenuate the dystrophic phenotype in mdx mice (model of DMD). Recently, an orally active ApN receptor agonist, AdipoRon, has been identified. This synthetic small molecule has the advantage of being more easily produced and administrable than ApN. The aim of this study was to investigate the potential effects of AdipoRon on the dystrophic muscle.

METHODS

Four-week-old mdx mice (n = 6-9 per group) were orally treated with AdipoRon (mdx-AR) for 8 weeks and compared with untreated (mdx) mice and to control (wild-type) mice. In vivo functional tests were carried out to measure the global force and endurance of mice. Ex vivo biochemical and molecular analyses were performed to evaluate the pathophysiology of the skeletal muscle. Finally, in vitro tests were conducted on primary cultures of healthy and DMD human myotubes.

RESULTS

AdipoRon treatment mitigated oxidative stress (-30% to 45% for 4-hydroxy-2-nonenal and peroxiredoxin 3, P < 0.0001) as well as inflammation in muscles of mdx mice (-35% to 65% for interleukin 1 beta, tumour necrosis factor alpha, and cluster of differentiation 68, a macrophage maker, P < 0.0001) while increasing the anti-inflammatory cytokine, interleukin 10 (~5-fold, P < 0.0001). AdipoRon also improved the myogenic programme as assessed by a ~2-fold rise in markers of muscle proliferation and differentiation (P < 0.01 or less vs. untreated mdx). Plasma lactate dehydrogenase and creatine kinase were reduced by 30-40% in mdx-AR mice, reflecting less sarcolemmal damage (P < 0.0001). When compared with untreated mdx mice, mdx-AR mice exhibited enhanced physical performance with an increase in both muscle force and endurance and a striking restoration of the running capacity during eccentric exercise. AdipoRon mainly acted through ApN receptor 1 by increasing AMP-activated protein kinase signalling, which led to repression of nuclear factor-kappa B, up-regulation of utrophin (a dystrophin analogue), and a switch towards an oxidative and more resistant fibre phenotype. The effects of AdipoRon were then recapitulated in human DMD myotubes.

CONCLUSIONS

These results demonstrate that AdipoRon exerts several beneficial effects on the dystrophic muscle. This molecule could offer promising therapeutic prospect for managing DMD or other muscle and inflammatory disorders.

Dysbacteriosis-induced LPS elevation disturbs the development of muscle progenitor cells by interfering with retinoic acid signaling

Whether myogenesis is affected by the maternal gut dysbacteriosis still remains ambiguous. In this study, first we show the elevated level of lipopolysaccharides (LPS) in a gut microbiota dysbiosis mouse model. Second, we demonstrate that the diameter of muscle fibers, limb development, and somitogenesis were inhibited in both the gut microbiota dysbiosis and LPS exposed mice and chicken embryos. These might be due to LPS disturbed the cell survival and key genes which regulate the somitogenesis and myogenesis. RNA sequencing and subsequent validation experiments verified that retinoic acid (RA) signaling perturbation was mainly responsible for the aberrant somite formation and differentiation. Subsequently, we found that LPS-induced reactive oxygen species (ROS generation and antioxidant genes such as Nrf2, AKR1B10) contributed to the above -mentioned interference with RA signaling. These findings highlight that the gut microbiota homeostasis is also involved in regulating the development of muscle progenitor cells during pregnancy.

The Switch from NF-YAl to NF-YAs Isoform Impairs Myotubes Formation

NF-YA, the regulatory subunit of the trimeric transcription factor (TF) NF-Y, is regulated by alternative splicing (AS) generating two major isoforms, “long” (NF-YAl) and “short” (NF-YAs). Muscle cells express NF-YAl. We ablated exon 3 in mouse C2C12 cells by a four-guide CRISPR/Cas9n strategy, obtaining clones expressing exclusively NF-YAs (C2-YAl-KO). C2-YAl-KO cells grow normally, but are unable to differentiate. Myogenin and—to a lesser extent, MyoD— levels are substantially lower in C2-YAl-KO, before and after differentiation. Expression of the fusogenic Myomaker and Myomixer genes, crucial for the early phases of the process, is not induced. Myomaker and Myomixer promoters are bound by MyoD and Myogenin, and Myogenin overexpression induces their expression in C2-YAl-KO. NF-Y inactivation reduces MyoD and Myogenin, but not directly: the Myogenin promoter is CCAAT-less, and the canonical CCAAT of the MyoD promoter is not bound by NF-Y in vivo. We propose that NF-YAl, but not NF-YAs, maintains muscle commitment by indirectly regulating Myogenin and MyoD expression in C2C12 cells. These experiments are the first genetic evidence that the two NF-YA isoforms have functionally distinct roles.

Skeletal Muscle Extracellular Matrix - What Do We Know About Its Composition, Regulation, and Physiological Roles? A Narrative Review

Skeletal muscle represents the largest body-composition component in humans. In addition to its primary function in the maintenance of upright posture and the production of movement, it also plays important roles in many other physiological processes, including thermogenesis, metabolism and the secretion of peptides for communication with other tissues. Research attempting to unveil these processes has traditionally focused on muscle fibers, i.e., the contractile muscle cells. However, it is a frequently overlooked fact that muscle fibers reside in a three-dimensional scaffolding that consists of various collagens, glycoproteins, proteoglycans, and elastin, and is commonly referred to as extracellular matrix (ECM). While initially believed to be relatively inert, current research reveals the involvement of ECM cells in numerous important physiological processes. In interaction with other cells, such as fibroblasts or cells of the immune system, the ECM regulates muscle development, growth and repair and is essential for effective muscle contraction and force transmission. Since muscle ECM is highly malleable, its texture and, consequently, physiological roles may be affected by physical training and disuse, aging or various diseases, such as diabetes. With the aim to stimulate increased efforts to study this still poorly understood tissue, this narrative review summarizes the current body of knowledge on (i) the composition and structure of the ECM, (ii) molecular pathways involved in ECM remodeling, (iii) the physiological roles of muscle ECM, (iv) dysregulations of ECM with aging and disease as well as (v) the adaptations of muscle ECM to training and disuse.

Muscle Progenitors Derived from Extraocular Muscles Express Higher Levels of Neurotrophins and their Receptors than other Cranial and Limb Muscles

Extraocular muscles (EOMs) show resistance to muscle dystrophies and sarcopenia. It has been recently demonstrated that they are endowed with different types of myogenic cells, all of which present an outstanding regenerative potential. Neurotrophins are important modulators of myogenic regeneration and act promoting myoblast proliferation, enhancing myogenic fusion rates and protecting myotubes from inflammatory stimuli. Here, we adapted the pre-plate cell isolation technique to obtain myogenic progenitors from the rat EOMs, and quantified their in vitro expression of neurotrophins and their receptors by RT–qPCR and immunohistochemistry, respectively. The results were compared with the expression on progenitors isolated from buccinator, tongue and limb muscles. Our quantitative analysis of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) transcripts showed, for the first time, that EOMs-derived cells express more of these factors and that they expressed TrkA, but not TrkB and TrkC receptors. On the contrary, the immunofluorescence analysis demonstrated high expression of p75^NTR on all myogenic progenitors, with the EOMs-derived cells showing higher expression. Taken together, these results suggest that the intrinsic trophic differences between EOMs-derived myogenic progenitors and their counterparts from other muscles could explain why those cells show higher proliferative and fusion rates, as well as better regenerative properties.

Reductive stress impairs myogenic differentiation

Myo-satellite cells regenerate and differentiate into skeletal muscle (SM) after acute or chronic injury. Changes in the redox milieu towards the oxidative arm at the wound site are known to compromise SM regeneration. Recently, we reported that abrogation of Nrf2/antioxidant signaling promotes oxidative stress and impairs SM regeneration in C57/Bl6 mice. Here, we investigated whether the activation of intracellular Nrf2 signaling favors antioxidant transcription and promotes myoblast differentiation. Satellite cell-like C2C12 myoblasts were treated with sulforaphane (SF; 1.0 & 5.0 μM) to activate Nrf2/antioxidant signaling during proliferation and differentiation (i.e. formation of myotubes/myofibers). SF-mediated Nrf2 activation resulted in increased expression of Nrf2-antioxidants (e.g. GCLC and G6PD) and augmented the production of reduced glutathione (GSH) leading to a reductive redox state. Surprisingly, this resulted in significant inhibition of myoblast differentiation, as observed from morphological changes and reduced expression of MyoD, Pax7, and Myh2, due to reductive stress (RS). Furthermore, supplementation of N-acetyl-cysteine (NAC) or GSH-ester or genetic knock-down of Keap1 (using siRNA) also resulted in RS-driven inhibition of differentiation. Interestingly, withdrawing Nrf2 activation rescued differentiation potential and formation of myotubes/myofibers from C2C12 myoblasts. Thus, abrogation of physiological ROS signaling through over-activation of Nrf2 (i.e. RS) and developing RS hampers differentiation of muscle satellite cells.

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Sulforaphane activates Nrf2 and establishes reductive stress (RS) in C2C12 myoblasts.

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RS abolishes basal ROS and significantly impede the differentiation of myoblasts.

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Augmentation of glutathione using pharmacological agents (NAC and GSH-ester) promotes RS and impairs differentiation.

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Precluding RS restores the myoblast differentiation process.

Combined Effect of Midazolam and Bone Morphogenetic Protein-2 for Differentiation Induction from C2C12 Myoblast Cells to Osteoblasts

In drug repositioning research, a new concept in drug discovery and new therapeutic opportunities have been identified for existing drugs. Midazolam (MDZ) is an anesthetic inducer used for general anesthesia. Here, we demonstrate the combined effects of bone morphogenetic protein-2 (BMP-2) and MDZ on osteogenic differentiation. An immortalized mouse myoblast cell line (C2C12 cell) was cultured in the combination of BMP-2 and MDZ (BMP-2+MDZ). The differentiation and signal transduction of C2C12 cells into osteoblasts were investigated at biological, immunohistochemical, and genetic cell levels. Mineralized nodules formed in C2C12 cells were characterized at the crystal engineering level. BMP-2+MDZ treatment decreased the myotube cell formation of C2C12 cells, and enhanced alkaline phosphatase activity and expression levels of osteoblastic differentiation marker genes. The precipitated nodules consisted of randomly oriented hydroxyapatite nanorods and nanoparticles. BMP-2+MDZ treatment reduced the immunostaining for both α1 and γ2 subunits antigens on the gamma-aminobutyric acid type A (GABAA) receptor in C2C12 cells, but enhanced that for BMP signal transducers. Our investigation showed that BMP-2+MDZ has a strong ability to induce the differentiation of C2C12 cells into osteoblasts and has the potential for drug repositioning in bone regeneration.

SQLE Promotes Differentiation and Apoptosis of Bovine Skeletal Muscle-Derived Mesenchymal Stem Cells

In this study, Squalene epoxidase (SQLE) overexpression vector was transfected into bovine skeletal muscle-derived mesenchymal stem/stromal cells (MSCs) to study the molecular mechanism of SQLE regulating meat quality through myogenesis. We initially profiled the expression of SQLE in cattle embryos and adults, in the muscle tissue of four different cattle varieties, and in 11 different tissues/organs of Guangxi cattle variety. Subsequently, we isolated and cultured bovine skeletal muscle-derived MSCs and detected the expression of SQLE during cell proliferation and differentiation. Then, we constructed a bovine SQLE overexpression vector and transfected it into bovine skeletal muscle-derived MSCs by liposome transfection. Cell Counting Kit-8 (CCK-8), 5-ethynyl-20-deoxyuridine (EdU), flow cytometry, immunofluorescence, and quantitative polymerase chain reaction assays were used to characterize cell proliferation and differentiation in detail. The results showed that the relative expression level of bovine SQLE gene in brain tissue was the highest, and in adult muscle tissue was significantly higher than that in embryonic stage. Especially, the expression of SQLE was significantly upregulated in cell differentiation stage. Furthermore, the proliferation, cell cycle, apoptosis, and myoblast differentiation assays indicated that SQLE significantly promoted the differentiation and apoptosis of bovine skeletal muscle-derived MSCs, but inhibited their proliferation. In conclusion, our study reveals the role of SQLE in myoblast differentiation. These results will provide new clues for the regulation network of bovine muscle development.

Long acting GLP-1 analog liraglutide ameliorates skeletal muscle atrophy in rodents

BACKGROUND

Skeletal muscle atrophy is characterized by muscle wasting with partial or complete functional loss. Skeletal muscle atrophy severely affects the quality of life and currently, there is no available therapy except for spinal muscular atrophy.

OBJECTIVE

Drug repositioning is a promising strategy that reduces cost and time due to prior availability of safety and toxicity details. Here we investigated myogenic and anti-atrophy effects of glucagon-like peptide-1 (GLP-1) analog liraglutide.

METHODS

We used several in vitro atrophy models in C2C12 cells and in vivo models in Sprague Dawley rats to study Liraglutide's efficacy. Western blotting was used to assess cAMP-dependent signaling pathways specifically activated by liraglutide. Therapeutic efficacy of liraglutide was investigated by histological analysis of transverse muscle sections followed by morphometry. Myogenic capacity was investigated by immunoblotting for myogenic factors.

RESULTS

Liraglutide induced myogenesis in C2C12 myoblasts through GLP-1 receptor via a cAMP-dependent complex network of signaling events involving protein kinase A, phosphoinositide 3-kinase/protein kinase B, p38 mitogen-activated protein kinase and extracellular signal-regulated kinase. Liraglutide imparted protection against freeze injury, denervation, and dexamethasone -induced skeletal muscle atrophy and improved muscular function in all these models. In a therapeutic model, liraglutide restored myofibrillar architecture in ovariectomy-induced atrophy. Anti-atrophy actions of liraglutide involved suppression of atrogene expression and enhancement in expression of myogenic factors.

CONCLUSION

Liraglutide imparted protection and restored myofibrillar architecture in diverse models of muscle atrophy. Given its potent anti-atrophy, and recently reported osteoanabolic effects, we propose liraglutide's clinical evaluation in skeletal muscle atrophy and musculoskeletal disorders associated with diverse pathologies.

Muscle regeneration in adiponectin knockout mice showed early activation of anti-inflammatory response with perturbations in myogenesis

Activation, proliferation, and differentiation of satellite cells can be influenced by extracellular factors, such as adiponectin. This adipokine has been proposed as a regulator of in vitro myogenesis, but its action on in vivo regeneration is not still elucidated. We used C57BL/6 (wild-type [WT]) and adiponectin knockout (AdKO) mice injured with barium chloride at periods of 3, 7, and 14 days after injury. The AdKO presented a higher number of centralized nuclei after 7 days, and a reduction in myogenic genes was observed after 3 days. Moreover, these mice presented an increase in anti-inflammatory cytokines after 3 and 7 days, and an increase in the M2 gene marker and proinflammatory cytokines after 7 days. The WT demonstrated an increase in adiponectin messenger RNA after 7 days. These results demonstrate that adiponectin is important in tissue remodeling during regeneration and that its deficiency does not compromise the maturation of muscle fibers, due to an increase in anti-inflammatory response; however, there is a possible impairment in proinflammatory response and an increase in centralized myonuclei.

Postnatal development of skeletal muscle in pigs with intrauterine growth restriction: morphofunctional phenotype and molecular mechanisms

Intrauterine growth restriction (IUGR) is a serious condition which impairs the achievement of the fetus' full growth potential and occurs in a natural and severe manner in pigs as a result of placental insufficiency. Reduced skeletal muscle mass in the fetus with IUGR persists into adulthood and may contribute to increased metabolic disease risk. To investigate skeletal muscle postnatal development, histomorphometrical patterns of the semitendinosus muscle, myosin heavy chain (MyHC; embryonic I, IIA, IIB and IIX isoforms) fiber composition and the relative expression of genes related to myogenesis, adipogenesis and growth during three specific periods: postnatal myogenesis (newborn to 100 days old), hypertrophy (100-150 days old), and postnatal development (newborn to 150 days old) were evaluated in female pigs with IUGR and normal birth weight (NW) female littermates. NW females presented higher body weights compared to their IUGR counterparts at all ages evaluated (P < 0.05). Moreover, growth restriction in utero affected the semitendinosus muscle weight, muscle fiber diameter, and muscle cross-sectional area, which were smaller in IUGR pigs at birth (P < 0.05). Notwithstanding the effects on muscle morphology, IUGR also affected muscle fiber composition, as the percentage of MyHC-I myofibers was higher at birth (P < 0.05), and, in 150-day-old gilts, a lower percentage of MyHC-IIX isoform (P < 0.05) and the presence of embryonic MyHC isoform were also observed. Regarding the pattern of gene expression in both the postnatal myogenesis and postnatal development periods, IUGR led to the downregulation of myogenic factors, which delayed skeletal muscle myogenesis (PAX7, MYOD, MYOG, MYF5 and DES). Altogether, growth restriction in utero affects muscle fiber number and size at birth and muscle fiber composition through the downregulation of myogenic factors, which determines the individual´s postnatal growth rate. This fact, associated with delayed myofiber development in growth-restricted animals, may affect meat quality characteristics in animal production. Hence, knowledge of the morphofunctional phenotype of the skeletal muscle throughout postnatal development in individuals with IUGR, and the mechanism that governs it, may provide a better understanding of the mechanisms that limit postnatal muscle growth, and help the establishment of potential strategies to improve muscle development and prevent the onset of later-life metabolic diseases.

Human and mouse skeletal muscle stem and progenitor cells in health and disease

The proper functioning of tissues and organs depends on their ability to self-renew and repair. Some of the tissues, like epithelia, renew almost constantly while in the others this process is induced by injury or diseases. The stem or progenitor cells responsible for tissue homeostasis have been identified in many organs. Some of them, such as hematopoietic or intestinal epithelium stem cells, are multipotent and can differentiate into various cell types. Others are unipotent. The skeletal muscle tissue does not self-renew spontaneously, however, it presents unique ability to regenerate in response to the injury or disease. Its repair almost exclusively relies on unipotent satellite cells. However, multiple lines of evidence document that some progenitor cells present in the muscle can be supportive for skeletal muscle regeneration. Here, we summarize the current knowledge on the complicated landscape of stem and progenitor cells that exist in skeletal muscle and support its regeneration. We compare the cells from two model organisms, i.e., mouse and human, documenting their similarities and differences and indicating methods to test their ability to undergo myogenic differentiation.

Divergent Regulation of Myotube Formation and Gene Expression by E2 and EPA during In-Vitro Differentiation of C2C12 Myoblasts

Estrogen (E2) and polyunsaturated fatty acids (n-3PUFA) supplements independently support general wellbeing and enhance muscle regeneration in-vivo and myotube formation in-vitro. However, the combined effect of E2 and n-3PUFA on myoblast differentiation is not known. The purpose of the study was to identify whether E2 and n-3PUFA possess a synergistic effect on in-vitro myogenesis. Mouse C2C12 myoblasts, a reliable model to reiterate myogenic events in-vitro, were treated with 10nM E2 and 50μM eicosapentaenoic acid (EPA) independently or combined, for 0–24 h or 0–120 h during differentiation. Immunofluorescence, targeted qPCR and next generation sequencing (NGS) were used to characterize morphological changes and differential expression of key genes involved in the regulation of myogenesis and muscle function pathways. E2 increased estrogen receptor α (Erα) and the expression of the mitogen-activated protein kinase 11 (Mapk11) within 1 h of treatment and improved myoblast differentiation and myotube formation. A significant reduction (p < 0.001) in myotube formation and in the expression of myogenic regulatory factors Mrfs (MyoD, Myog and Myh1) and the myoblast fusion related gene, Tmem8c, was observed in the presence of EPA and the combined E2/EPA treatment. Additionally, EPA treatment at 48 h of differentiation inhibited the majority of genes associated with the myogenic and striated muscle contraction pathways. In conclusion, EPA and E2 had no synergistic effect on myotube formation in-vitro. Independently, EPA inhibited myoblast differentiation and overrides the stimulatory effect of E2 when used in combination with E2.

Systemic Actions of Breast Cancer Facilitate Functional Limitations

Breast cancer is a disease of a specific organ, but its effects are felt throughout the body. The systemic effects of breast cancer can lead to functional limitations in patients who suffer from muscle weakness, fatigue, pain, fibromyalgia, or many other dysfunctions, which hasten cancer-associated death. Mechanistic studies have identified quite a few molecular defects in skeletal muscles that are associated with functional limitations in breast cancer. These include circulating cytokines such as TNF-α, IL-1, IL-6, and TGF-β altering the levels or function of myogenic molecules including PAX7, MyoD, and microRNAs through transcriptional regulators such as NF-κB, STAT3, and SMADs. Molecular defects in breast cancer may also include reduced muscle mitochondrial content and increased extracellular matrix deposition leading to energy imbalance and skeletal muscle fibrosis. This review highlights recent evidence that breast cancer-associated molecular defects mechanistically contribute to functional limitations and further provides insights into therapeutic interventions in managing functional limitations, which in turn may help to improve quality of life in breast cancer patients.

The metabolic sensor PASK is a histone 3 kinase that also regulates H3K4 methylation by associating with H3K4 MLL2 methyltransferase complex

The metabolic sensor Per-Arnt-Sim (Pas) domain-containing serine/threonine kinase (PASK) is expressed predominantly in the cytoplasm of different cell types, although a small percentage is also expressed in the nucleus. Herein, we show that the nuclear PASK associates with the mammalian H3K4 MLL2 methyltransferase complex and enhances H3K4 di- and tri-methylation. We also show that PASK is a histone kinase that phosphorylates H3 at T3, T6, S10 and T11. Taken together, these results suggest that PASK regulates two different H3 tail modifications involving H3K4 methylation and H3 phosphorylation. Using muscle satellite cell differentiation and functional analysis after loss or gain of Pask expression using the CRISPR/Cas9 system, we provide evidence that some of the regulatory functions of PASK during development and differentiation may occur through the regulation of these histone modifications.

The factors present in regenerating muscles impact bone marrow-derived mesenchymal stromal/stem cell fusion with myoblasts

BACKGROUND

Satellite cells, a population of unipotent stem cells attached to muscle fibers, determine the excellent regenerative capability of injured skeletal muscles. Myogenic potential is also exhibited by other cell populations, which exist in the skeletal muscles or come from other niches. Mesenchymal stromal/stem cells inhabiting the bone marrow do not spontaneously differentiate into muscle cells, but there is some evidence that they are capable to follow the myogenic program and/or fuse with myoblasts.

METHODS

In the present study we analyzed whether IGF-1, IL-4, IL-6, and SDF-1 could impact human and porcine bone marrow-derived mesenchymal stromal/stem cells (hBM-MSCs and pBM-MSCs) and induce expression of myogenic regulatory factors, skeletal muscle-specific structural, and adhesion proteins. Moreover, we investigated whether these factors could induce both types of BM-MSCs to fuse with myoblasts. IGF-1, IL-4, IL-6, and SDF-1 were selected on the basis of their role in embryonic myogenesis as well as skeletal muscle regeneration.

RESULTS

We found that hBM-MSCs and pBM-MSCs cultured in vitro in the presence of IGF-1, IL-4, IL-6, or SDF-1 did not upregulate myogenic regulatory factors. Consequently, we confirmed the lack of their naïve myogenic potential. However, we noticed that IL-4 and IL-6 impacted proliferation and IL-4, IL-6, and SDF-1 improved migration of hBM-MSCs. IL-4 treatment resulted in the significant increase in the level of mRNA encoding CD9, NCAM, VCAM, and m-cadherin, i.e., proteins engaged in cell fusion during myotube formation. Additionally, the CD9 expression level was also driven by IGF-1 treatment. Furthermore, the pre-treatment of hBM-MSCs either with IGF-1, IL-4, or SDF-1 and treatment of pBM-MSCs either with IGF-1 or IL-4 increased the efficacy of hybrid myotube formation between these cells and C2C12 myoblasts.

CONCLUSIONS

To conclude, our study revealed that treatment with IGF-1, IL-4, IL-6, or SDF-1 affects BM-MSC interaction with myoblasts; however, it does not directly promote myogenic differentiation of these cells.

Lipid accumulation and mitochondrial abnormalities are associated with fiber atrophy in the skeletal muscle of rats with collagen-induced arthritis

Rheumatoid arthritis (RA) has a negative impact on muscle mass, and reduces patient's mobility and autonomy. Furthermore, RA is associated with metabolic comorbidities, notably in lipid homeostasis by unknown mechanisms. To understand the links between the loss in muscle mass and the metabolic abnormalities, arthritis was induced in male Sprague Dawley rats (n = 11) using the collagen-induced arthritis model. Rats immunized with bovine type II collagen were compared to a control group of animals (n = 11) injected with acetic acid and complete Freund's adjuvant. The clinical severity of the ensuing arthritis was evaluated weekly by a semi-quantitative score. Skeletal muscles from the hind limb were used for the histological analysis and exploration of mitochondrial activity, lipid accumulation, metabolism and regenerative capacities. A significant atrophy in tibialis anterior muscle fibers was observed in the arthritic rats despite a non-significant decrease in the weight of the muscles. Despite moderate inflammation, accumulation of triglycerides (P < 0.05), reduced mitochondrial DNA copy number (P < 0.05) and non-significant dysfunction in mitochondrial cytochrome c oxidase activity were found in the gastrocnemius muscle. Concomitantly, our results suggested an activation of the muscle specific E3 ubiquitin ligases MuRF-1 and MAFbx. Finally, the adipose tissue from the arthritic rats exhibited decreased PPARγ mRNA suggesting reduced adipogenic capacities. In conclusion, the reduced adipose tissue adipogenic capacity and skeletal muscle mitochondrial capacity are probably involved in the activation of protein catabolism, inhibition of myogenesis, accumulation of lipids and fiber atrophy in the skeletal muscle during RA.

Methylmercury modifies temporally expressed myogenic regulatory factors to inhibit myoblast differentiation

Methylmercury (MeHg) is a pervasive environmental toxicant, with known detrimental effects on neurodevelopment. Despite a longstanding paradigm of neurotoxicity, where motor deficits are prevalent among those developmentally exposed, consideration of muscle as a MeHg target has received minimal investigation. Recent evidence has identified muscle-specific gene networks that modulate developmental sensitivity to MeHg toxicity. One such network is muscle cell differentiation. Muscle cell differentiation is a coordinated process regulated by the myogenic regulatory factors (MRFs): Myf5, MyoD, MyoG, and MRF4. A previous study demonstrated that MeHg inhibits muscle cell differentiation in vitro, concurrent with reduced MyoG expression. The potential for MeHg to modify the temporal expression of the MRFs to alter differentiation, however, has yet to be fully explored. Using the C2C12 mouse myoblast model, we examined MRF expression profiles at various stages subsequent to MeHg exposure to proliferating myoblasts. MeHg was seen to persistently alter myoblast differentiation capacity, as myod, myog, and mrf4 gene expression were all affected. Myog exhibited the most robust changes in expression across the various culture conditions, while myf5 was unaffected. Following MeHg exposure to myoblasts, where elevated p21 expression indicated departure from proliferation, cells failed to subsequently differentiate, even in the absence of MeHg, as reflected by a concurrent reduction in MRF4 and myosin heavy chain (MHC), markers of terminal differentiation. Our results indicate that within a brief window of exposure MeHg can disrupt the intrinsic myogenic differentiation program of proliferative myoblasts.

MicroRNA-206 regulates cell proliferation by targeting G6PD in skeletal muscle

Skeletal muscle is a major component of body mass and plays a central role in the control of whole-body metabolism in humans and animals. Therefore, elucidation of the underlying mechanisms of skeletal growth and development are expected to lead to the discovery of novel genes and pathways related to muscle disease. miR-206, a skeletal muscle-specific microRNA, plays a crucial role in myogenesis; however, miR-206 is known to function in myogenic differentiation, whether or not it affects muscle cells' proliferation, and the underlying mechanisms are unknown. In this study, we investigated the effect of miR-206 on muscle cell proliferation and differentiation, as well as its effect on myofiber type conversion using mouse C2C12 myoblasts. The results showed that overexpression of miR-206 inhibited cell proliferation and promoted muscle cell differentiation, but it did not affect myofiber type conversion. Intriguingly, we found that overexpression of miR-206 suppressed muscle cell proliferation and induced cell cycle arrest in G₀/G₁ phase by inhibiting the glucose-6-phosphate dehydrogenase (G6PD) gene. Taken together, we demonstrated that the miR-206-G6PD pathway suppresses muscle cell proliferation, and these findings may facilitate the treatment of muscle diseases.-Jiang, A., Dong, C., Li, B., Zhang, Z., Chen, Y., Ning, C., Wu, W., Liu, H. MicroRNA-206 regulates cell proliferation by targeting G6PD in skeletal muscle.

Interleukin-6 Induces Myogenic Differentiation via JAK2-STAT3 Signaling in Mouse C2C12 Myoblast Cell Line and Primary Human Myoblasts

Postnatal muscle growth and exercise- or injury-induced regeneration are facilitated by myoblasts. Myoblasts respond to a variety of proteins such as cytokines that activate various signaling cascades. Cytokines belonging to the interleukin 6 superfamily (IL-6) influence myoblasts' proliferation but their effect on differentiation is still being researched. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is one of the key signaling pathways identified to be activated by IL-6. The aim of this study was to investigate myoblast fate as well as activation of JAK-STAT pathway at different physiologically relevant IL-6 concentrations (10 pg/mL; 100 pg/mL; 10 ng/mL) in the C2C12 mouse myoblast cell line and primary human myoblasts, isolated from eight young healthy male volunteers. Myoblasts' cell cycle progression, proliferation and differentiation in vitro were assessed. Low IL-6 concentrations facilitated cell cycle transition from the quiescence/Gap1 (G0/G1) to the synthesis (S-) phases. Low and medium IL-6 concentrations decreased the expression of myoblast determination protein 1 (MyoD) and myogenin and increased proliferating cell nuclear antigen (PCNA) expression. In contrast, high IL-6 concentration shifted a larger proportion of cells to the pro-differentiation G0/G1 phase of the cell cycle, substantiated by significant increases of both MyoD and myogenin expression and decreased PCNA expression. Low IL-6 concentration was responsible for prolonged JAK1 activation and increased suppressor of cytokine signaling 1 (SOCS1) protein expression. JAK-STAT inhibition abrogated IL-6-mediated C2C12 cell proliferation. In contrast, high IL-6 initially increased JAK1 activation but resulted in prolonged JAK2 activation and elevated SOCS3 protein expression. High IL-6 concentration decreased interleukin-6 receptor (IL-6R) expression 24 h after treatment whilst low IL-6 concentration increased IL-6 receptor (IL-6R) expression at the same time point. In conclusion, this study demonstrated that IL-6 has concentration- and time-dependent effects on both C2C12 mouse myoblasts and primary human myoblasts. Low IL-6 concentration induces proliferation whilst high IL-6 concentration induces differentiation. These effects are mediated by specific components of the JAK/STAT/SOCS pathway.

Novel lncRNA lncFAM200B: Molecular Characteristics and Effects of Genetic Variants on Promoter Activity and Cattle Body Measurement Traits

Skeletal muscle is one of the three major muscle types in an organism and has key roles in the motor system, metabolism, and homeostasis. RNA-Seq analysis showed that novel lncRNA, lncFAM200B, was differentially expressed in embryonic, neonatal, and adult cattle skeletal muscles. The main aim of this study was to investigate the molecular and expression characteristics of lncFAM200B along with its crucial genetic variations. Our results showed that bovine lncFAM200B was a 472 nucleotide (nt) non-coding RNA containing two exons. The transcription factor binding site prediction analysis found that lncFAM200B promoter region was enriched with SP1 transcription factor, which promotes the binding of myogenic regulatory factor MyoD and DNA sequence. The mRNA expression analysis showed that lncFAM200B was differentially expressed in embryonic, neonatal, adult bovine muscle tissues, and the lncFAM200B expression trend positively correlated with that of MyoG and Myf5 in myoblast proliferation and differential stages. To identify the promoter active region of lncFAM200B, we constructed promoter luciferase reporter gene vector pGL3-Basic plasmids containing lncFAM200B promoter sequences and transfected them into 293T, C2C12, and 3T3-L1 cells. Our results suggested that lncFAM200B promoter active region was from −403 to −139 (264 nt) of its transcription start site, covering 6 SP1 potential binding sites. Furthermore, we found a novel C-T variation, named as SNP2 (ERZ990081 in European Variation Archive) in the promoter active region, which was linked to the nearby SNP1 (rs456951291 in Ensembl database). The genotypes of SNP1 and combined genotypes of SNP1 and SNP2 were significantly associated with Jinnan cattle hip height. The luciferase activity analysis found that the SNP1-SNP2 haplotype CC had the highest luciferase activity, which was consistent with the association analysis result that the combined genotype CC-CC carriers had the highest hip height in Jinnan cattle. In conclusion, our data showed that lncFAM200B is a positive regulator of muscle development and that SNP1 and SNP2 could be used as genetic markers for marker-assisted selection (MAS) breeding of beef cattle.

Defective angiogenesis in CXCL12 mutant mice impairs skeletal muscle regeneration

BACKGROUND

During muscle regeneration, the chemokine CXCL12 (SDF-1) and the synthesis of some specific heparan sulfates (HS) have been shown to be critical. CXCL12 activity has been shown to be heavily influenced by its binding to extracellular glycosaminoglycans (GAG) by modulating its presentation to its receptors and by generating haptotactic gradients. Although CXCL12 has been implicated in several phases of tissue repair, the influence of GAG binding under HS influencing conditions such as acute tissue destruction remains understudied.

METHODS

To investigate the role of the CXCL12/HS proteoglycan interactions in the pathophysiology of muscle regeneration, we performed two models of muscle injuries (notexin and freeze injury) in mutant CXCL12^Gagtm/Gagtm mice, where the CXCL12 gene having been selectively mutated in critical binding sites of CXCL12 to interact with HS. Histological, cytometric, functional transcriptomic, and ultrastructure analysis focusing on the satellite cell behavior and the vessels were conducted on muscles before and after injuries. Unless specified, statistical analysis was performed with the Mann-Whitney test.

RESULTS

We showed that despite normal histology of the resting muscle and normal muscle stem cell behavior in the mutant mice, endothelial cells displayed an increase in the angiogenic response in resting muscle despite the downregulated transcriptomic changes induced by the CXCL12 mutation. The regenerative capacity of the CXCL12-mutated mice was only delayed after a notexin injury, but a severe damage by freeze injury revealed a persistent defect in the muscle regeneration of CXCL12 mutant mice associated with vascular defect and fibroadipose deposition with persistent immune cell infiltration.

CONCLUSION

The present study shows that CXCL12 is crucial for proper muscle regeneration. We highlight that this homing molecule could play an important role in drastic muscle injuries and that the regeneration defect could be due to an impairment of angiogenesis, associated with a long-lasting fibro-adipogenic scar.

Regulation of present and future development by maternal regulatory signals acting on the embryo during the morula to blastocyst transition - insights from the cow

The preimplantation embryo has a remarkable ability to execute its developmental program using regulatory information inherent within itself. Nonetheless, the uterine environment is rich in cell signaling molecules termed embryokines that act on the embryo during the morula-to-blastocyst transition, promoting blastocyst formation and programming the embryo for subsequent developmental events. Programming can not only affect developmental processes important for continuance of development in utero but also affect characteristics of the offspring during postnatal life. Given the importance of embryokines for regulation of embryonic development, it is likely that some causes of infertility involve aberrant secretion of embryokines by the uterus. Embryokines found to regulate development of the bovine embryo include insulin-like growth factor 1, colony stimulating factor 2 (CSF2), and dickkopf WNT signaling pathway inhibitor 1. Embryo responses to CSF2 exhibit sexual dimorphism, suggesting that sex-specific programming of postnatal function is caused by maternal signals acting on the embryo during the preimplantation period that regulate male embryos differently than female embryos.

Zinc finger protein 593 is upregulated during skeletal muscle atrophy and modulates muscle cell differentiation

Skeletal muscle atrophy is a debilitating condition that can arise due to aging, cancer, corticosteroid use, and denervation. To better characterize the molecular genetic events of neurogenic atrophy, a previous study analyzed gene expression patterns in gastrocnemius muscle following sciatic nerve transection and found for the first time that Zinc Finger Protein 593 (Zfp593) is expressed in skeletal muscle and is induced in response to denervation. Quantitative PCR and Western blot analyses confirmed that Zfp593 is expressed in both proliferating myoblasts and differentiated myotubes. To assess sub-cellular location, GFP-tagged Zfp593 was expressed in C₂C₁₂ cells and found to localize to the nucleus. The Zfp593 protein possesses a putative zinc finger domain and is believed to function as a modulator of the Oct-2 transcription factor. Interestingly, ectopic expression of Zfp593 did not affect the ability of Oct-1 or Oct-2 to inhibit an Oct reporter gene in muscle cells. Finally, Zfp593 overexpression in cultured muscle cells resulted in significant repression of muscle cell differentiation and attenuation of ERK1/2 and p38 phosphorylation, but did not vitiate protein synthesis. The discovery that Zfp593 is expressed in skeletal muscle combined with the observation that it is induced in response to neurogenic atrophy furthers our understanding of the molecular genetic events of muscle wasting.

Muscle satellite cells and impaired late stage regeneration in different murine models for muscular dystrophies

Satellite cells (SCs) are the main muscle stem cells responsible for its regenerative capacity. In muscular dystrophies, however, a failure of the regenerative process results in muscle degeneration and weakness. To analyze the effect of different degrees of muscle degeneration in SCs behavior, we studied adult muscle of the dystrophic strains: DMDmdx, Largemyd, DMDmdx/Largemyd, with variable histopathological alterations. Similar results were observed in the dystrophic models, which maintained normal levels of PAX7 expression, retained the Pax7-positive SCs pool, and their proliferation capacity. Moreover, elevated expression of MYOG, an important myogenic factor, was also observed. The ability to form new fibers was verified by the presence of dMyHC positive regenerating fibers. However, those fibers had incomplete maturation characteristics, such as small and homogenous fiber caliber, which could contribute to their dysfunction. We concluded that dystrophic muscles, independently of their degeneration degree, retain their SCs pool with proliferating and regenerative capacities. Nonetheless, the maturation of these new fibers is incomplete and do not prevent muscle degeneration. Taken together, these results suggest that the improvement of late muscle regeneration should better contribute to therapeutic approaches.

Epigenomic mechanisms of alcohol-induced impaired differentiation of skeletal muscle stem cells; role of Class IIA histone deacetylases

Loss of functional metabolic muscle mass remains a strong and consistent predictor of mortality among people living with human immunodeficiency virus (PLWH). PLWH have a higher incidence of alcohol use disorder (AUD), and myopathy is a significant clinical comorbidity due to AUD. One mechanism of skeletal muscle (SKM) mass maintenance and repair is by differentiation and fusion of satellite cells (SCs) to existing myofibers. Previous studies demonstrated that chronic binge alcohol (CBA) administration decreases SC differentiation potential, myogenic gene expression, and miR-206 expression in simian immunodeficiency virus (SIV)-infected male rhesus macaques and that miR-206 targets the Class IIA histone deacetylase, HDAC4. The aim of this study was to determine whether alcohol-induced increases in Class IIA HDACs mediate the observed decrease in differentiation potential of SCs. Data show that CBA dysregulated HDAC gene expression in SKM and myoblasts of SIV-infected macaques. CBA and antiretroviral therapy increased HDAC activity in SKM and this was positively correlated with HDAC4 gene expression. In vitro ethanol (ETOH) treatment increased HDAC expression during differentiation and decreased differentiation potential of myoblasts. HDAC expression was negatively correlated with fusion index and myotube formation, indicators of differentiation potential. Treatment with a Class II HDAC inhibitor, TMP195, restored differentiation in ETOH-treated myoblasts. MEF2C expression at day 3 of differentiation was positively correlated with fusion index and myotube formation. These findings suggest that an alcohol-mediated increase in Class IIA HDAC expression contributes to decreased myoblast differentiation by downregulating MEF2C, a transcription factor critical for myogenesis.

Expression and role of lncRNAs in the regeneration of skeletal muscle following contusion injury

Studies performed previously have indicated that long non-coding RNAs (lncRNAs) may be involved in skeletal muscle regeneration; however, the roles of lncRNAs during the repair of skeletal muscle contusion remain unclear. The present study established a mouse skeletal muscle contusion injury model to identify the roles of lncRNAs that are specifically enriched in the skeletal muscle, namely metastasis-associated lung adenocarcinoma transcript 1 (Malat1), H19, myogenesis-associated lnc (lnc-mg), long intergenic non-protein coding RNAs (linc)-muscle differentiation 1 (linc-MD1), linc-yin yang 1 (linc-YY1) and sirtuin 1-antisense (Sirt1-AS). Morphological analyses revealed that fibrotic scars and regenerating myofibers were formed in the muscle following contusion injury. Gene expression was analyzed by reverse transcription-quantitative polymerase chain reaction. The data revealed that the expression of inflammatory cytokines, myogenic regulatory factors and angiogenic factors increased significantly following skeletal muscle contusion. Additionally, various lncRNAs, including Malat1, H19, lnc-mg, linc-MD1, linc-YY1 and Sirt1-AS were also upregulated. Correlation was also observed between lncRNAs and regulatory factors for skeletal muscle regeneration including transforming growth factor-β1, myogenic differentiation, myogenin, myogenic factor 5 (myf5), myf6, hypoxia-inducible factor-1α and angiopoietin 1. In conclusion, lncRNAs may serve important roles in the regeneration of skeletal muscle following contusion injury, which provides a promising therapy avenue for muscle injury.

Master control: transcriptional regulation of mammalian Myod

MYOD is a master regulator of the skeletal myogenic program. But what regulates expression of Myod? More than 20 years ago, studies established that Myod expression is largely controlled by just two enhancer regions located within a region 24 kb upstream of the transcription start site in mammals, which regulate Myod expression in the embryo, fetus and adult. Despite this apparently simple arrangement, Myod regulation is complex, with different combinations of transcription factors acting on these enhancers in different muscle progenitor cells and phases of differentiation. A range of epigenetic modifications in the Myod upstream region also play a part in activating and repressing Myod expression during development and regeneration. Here the evidence for this binding at Myod control regions is summarized, giving an overview of our current understanding of Myod expression regulation in mammals.

KLF2 in Myeloid Lineage Cells Regulates the Innate Immune Response during Skeletal Muscle Injury and Regeneration

Skeletal muscle repair and regeneration after injury requires coordinated interactions between the innate immune system and the injured muscle. Myeloid cells predominate in these interactions. This study examined the role of KLF2, a zinc-finger transcription factor that regulates immune cell activation, in specifying myeloid cell functions during muscle regeneration. Loss of KLF2 in myeloid lineage cells (myeKlf2^-/- mice) dramatically enhanced the initial inflammatory response to acute muscle injury (cardiotoxin). Injured muscles showed dramatically elevated expression of inflammatory mediators and greater numbers of infiltrating, pro-inflammatory monocytes that matured earlier into activated macrophages. Notably, the inflammatory phase resolved earlier and regeneration progressed to myogenesis, marked by elevated expression of factors that promote the formation of new fibers from satellite cells. Regeneration was completed earlier, with phenotypically normal adult fibers integrated into the muscle syncytium. These findings identify myeloid KLF2 as a key regulator of myeloid cell functions in adult skeletal muscle regeneration.

Long-Term Cryopreservation and Revival of Tissue-Engineered Skeletal Muscle

IMPACT STATEMENT

The ability to freeze, revive, and prolong the lifetime of tissue-engineered skeletal muscle without incurring any loss of function represents a significant advancement in the field of tissue engineering. Cryopreservation enables the efficient fabrication, storage, and shipment of these tissues. This in turn facilitates multidisciplinary collaboration between research groups, enabling advances in skeletal muscle regenerative medicine, organ-on-a-chip models of disease, drug testing, and soft robotics. Furthermore, the observation that freezing undifferentiated skeletal muscle enhances functional performance may motivate future studies developing stronger and more clinically relevant engineered muscle.

Somite development in the avian tail

Somites are epithelial segments of the paraxial mesoderm. Shortly after their formation, the epithelial somites undergo extensive cellular rearrangements and form specific somite compartments, including the sclerotome and the myotome, which give rise to the axial skeleton and to striated musculature, respectively. The dynamics of somite development varies along the body axis, but most research has focused on somite development at thoracolumbar levels. The development of tail somites has not yet been thoroughly characterized, even though vertebrate tail development has been intensely studied recently with respect to the termination of segmentation and the limitation of body length in evolution. Here, we provide a detailed description of the somites in the avian tail from the beginning of tail formation at HH-stage 20 to the onset of degeneration of tail segments at HH-stage 27. We characterize the formation of somite compartment formation in the tail region with respect to morphology and the expression patterns of the sclerotomal marker gene paired-box gene 1 (Pax1) and the myotomal marker genes MyoD and myogenic factor 5 (Myf5). Our study gives insight into the development of the very last segments formed in the avian embryo, and provides a basis for further research on the development of tail somite derivatives such as tail vertebrae, pygostyle and tail musculature.

Primary myogenesis in the sand lizard (Lacerta agilis) limb bud

Our studies conducted on reptilian limb muscle development revealed, for the first time, early forelimb muscle differentiation at the morphological and molecular level. Sand lizard skeletal muscle differentiation in the early forelimb bud was investigated by light, confocal, and transmission electron microscopy as well as western blot. The early forelimb bud, filled with mesenchymal cells, is surrounded by monolayer epithelium cells. The immunocytochemical analysis revealed the presence of Pax3- and Lbx-positive cells in the vicinity of the ventro-lateral lip (VLL) of the dermomyotome, suggesting that VLL is the source of limb muscle progenitor cells. Furthermore, Pax3- and Lbx-positive cells were observed in the dorsal and ventral myogenic pools of the forelimb bud. Skeletal muscle development in the early limb bud is asynchronous, which is manifested by the presence of myogenic cells in different stages of differentiation: multinucleated myotubes with well-developed contractile apparatus, myoblasts, and mitotically active premyoblasts. The western blot analysis revealed the presence of MyoD and Myf5 proteins in all investigated developmental stages. The MyoD western blot analysis showed two bands corresponding to monomeric (mMyoD) and dimeric (dMyoD) fractions. Two separate bands were also detected in the case of Myf5. The observed bands were related to non-phosphorylated (Myf5) and phosphorylated (pMyf5) fractions of Myf5. Our investigations on sand lizard forelimb myogenesis showed that the pattern of muscle differentiation in the early forelimb bud shares many features with rodents and chicks.

MiR-501-3p Forms a Feedback Loop with FOS, MDFI, and MyoD to Regulate C2C12 Myogenesis

Skeletal muscle plays an essential role in maintaining body energy homeostasis and body flexibility. Loss of muscle mass leads to slower wound healing and recovery from illness, physical disability, poor quality of life, and higher health care costs. So, it is critical for us to understand the mechanism of skeletal muscle myogenic differentiation for maintaining optimal health throughout life. miR-501-3p is a novel muscle-specific miRNA, and its regulation mechanism on myoblast myogenic differentiation is still not clear. We demonstrated that FOS was a direct target gene of miR-501-3p, and MyoD regulated miR-501-3p host gene Clcn5 through bioinformatics prediction. Our previous laboratory experiment found that MDFI overexpression promoted C2C12 myogenic differentiation and MyoD expression. The database also showed there is an FOS binding site in the MDFI promoter region. Therefore, we hypothesize that miR-501-3p formed a feedback loop with FOS, MDFI, and MyoD to regulate myoblast differentiation. To validate our hypothesis, we demonstrated miR-501-3p function in the proliferation and differentiation period of C2C12 cells by transfecting cells with miR-501-3p mimic and inhibitor. Then, we confirmed there is a direct regulatory relationship between miR-501-3p and FOS, MyoD and miR-501-3p, FOS and MDFI through QPCR, dual-luciferase reporter system, and ChIP experiments. Our results not only expand our understanding of the muscle myogenic development mechanism in which miRNA and genes participate in controlling skeletal muscle development, but also provide treatment strategies for skeletal muscle or metabolic-related diseases in the future.

Effect of Testosterone on FGF2, MRF4, and Myostatin in Hypogonadotropic Hypogonadism: Relevance to Muscle Growth

CONTEXT

Fibroblast growth factor (FGF)2 is an important stimulatory modulator of satellite cells in skeletal muscle. Satellite cells play a cardinal role in muscle growth and repair.

OBJECTIVE

We evaluated whether skeletal muscle expression of FGF2 and muscle growth and differentiation factors are reduced in patients with hypogonadotropic hypogonadism (HH) and whether testosterone replacement therapy results in their restoration.

DESIGN

This is a secondary analysis of a previously completed trial of testosterone replacement in men with type 2 diabetes and HH.

SETTING

Clinical Research Center at a university.

PATIENTS

Twenty-two men with HH and 20 eugonadal men were compared at baseline.

INTERVENTIONS

Twelve men with HH were treated with intramuscular injections of 250 mg testosterone every 2 weeks for 22 weeks, and 10 men received placebo injections. Quadriceps muscle biopsies and blood samples were obtained before and after testosterone therapy.

OUTCOME MEASURES AND RESULTS

The expression of FGF2 and FGF receptor (FGFR)2 in skeletal muscle of men with HH was significantly lower than that in eugonadal men by 57% and 39%, respectively (P < 0.05). After 22 weeks of testosterone, the expression of FGF2 increased, whereas that of myogenic regulatory factor (MRF)4 and myostatin decreased significantly. There was no change in expression of FGFR2, myogenin, or myogenic differentiation protein in the skeletal muscle. Plasma FGF2 and IGF-1 concentrations increased after testosterone therapy.

CONCLUSIONS

These data show that testosterone is a major modulator of FGF2, MRF4, and myostatin expression in skeletal muscle. These effects may contribute to the increase in muscle mass after testosterone therapy.

MicroRNA therapy stimulates uncontrolled cardiac repair after myocardial infarction in pigs

Prompt coronary catheterization and revascularization have dramatically improved the outcome of myocardial infarction, but also have resulted in a growing number of survived patients with permanent structural damage of the heart, which frequently leads to heart failure. Finding new treatments for this condition is a largely unmet clinical need 1, especially because of the incapacity of cardiomyocytes to replicate after birth and thus achieve regeneration of the lost contractile tissue 2. Here we show that expression of human microRNA-199a in infarcted pig hearts is capable of stimulating cardiac repair. One month after myocardial infarction and delivery of this microRNA through an adeno-associated viral vector, the treated animals showed marked improvements in both global and regional contractility, increased muscle mass and reduced scar size. These functional and morphological findings correlated with cardiomyocyte de-differentiation and proliferation. At longer follow-up, however, persistent and uncontrolled expression of the microRNA resulted in sudden arrhythmic death of most of the treated pigs. Such events were concurrent with myocardial infiltration of proliferating cells displaying a poorly differentiated myoblastic phenotype. These results show that achieving cardiac repair through the stimulation of endogenous cardiomyocyte proliferation is attainable in large mammals, however this therapy needs to be tightly dosed.

Myostatin gene promoter: structure, conservation and importance as a target for muscle modulation

Myostatin (MSTN) is one of the key factors regulating myogenesis. Because of its role as a negative regulator of muscle mass deposition, much interest has been given to its protein and, in recent years, several studies have analysed MSTN gene regulation. This review discusses the MSTN gene promoter, focusing on its structure in several animal species, both vertebrate and invertebrate. We report the important binding sites considering their degree of phylogenetic conservation and roles they play in the promoter activity. Finally, we discuss recent studies focusing on MSTN gene regulation via promoter manipulation and the potential applications they have both in medicine and agriculture.

MiR-34b-5p Mediates the Proliferation and Differentiation of Myoblasts by Targeting IGFBP2

As key post-transcriptional regulators, microRNAs (miRNAs) play an indispensable role in skeletal muscle development. Our previous study suggested that miR-34b-5p and IGFBP2 could have a potential role in skeletal muscle growth. Our goal in this study is to explore the function and regulatory mechanism of miR-34b-5p and IGFBP2 in myogenesis. In this study, the dual-luciferase reporter assay and Western blot analysis showed that IGFBP2 is a direct target of miR-34b-5p. Flow cytometric analysis and EdU assay showed that miR-34b-5p could repress the cell cycle progression of myoblasts, and miR-34b-5p could promote the formation of myotubes by promoting the expression of MyHC. On the contrary, the overexpression of IGFBP2 significantly facilitated the proliferation of myoblasts and hampered the formation of myotubes. Together, our results indicate that miR-34b-5p could mediate the proliferation and differentiation of myoblasts by targeting IGFBP2.

RNA-sequencing reveals altered skeletal muscle contraction, E3 ligases, autophagy, apoptosis, and chaperone expression in patients with critical illness myopathy

Background

Critical illness myopathy (CIM) is associated with severe skeletal muscle wasting and impaired function in intensive care unit (ICU) patients. The mechanisms underlying CIM remain incompletely understood. To elucidate the biological activities occurring at the transcriptional level in the skeletal muscle of ICU patients with CIM, the gene expression profiles, potential upstream regulators, and enrichment pathways were characterized using RNA sequencing (RNA-seq). We also compared the skeletal muscle gene signatures in ICU patients with CIM and genes perturbed by mechanical loading in one leg of the ICU patients, with an aim of reducing the loss of muscle function.

Methods

RNA-seq was used to assess gene expression changes in tibialis anterior skeletal muscle samples from seven critically ill, immobilized, and mechanically ventilated ICU patients with CIM and matched control subjects. We also examined skeletal muscle gene expression for both legs of six ICU patients with CIM, where one leg was mechanically loaded for 10 h/day for an average of 9 days.

Results

In total, 6257 of 17,221 detected genes were differentially expressed (84% upregulated; p < 0.05 and fold change ≥ 1.5) in skeletal muscle from ICU patients with CIM when compared to control subjects. The differentially expressed genes were highly associated with gene changes identified in patients with myopathy, sepsis, long-term inactivity, polymyositis, tumor, and repeat exercise resistance. Upstream regulator analysis revealed that the CIM signature could be a result of the activation of MYOD1, p38 MAPK, or treatment with dexamethasone. Passive mechanical loading only reversed expression of 0.74% of the affected genes (46 of 6257 genes).

Conclusions

RNA-seq analysis revealed that the marked muscle atrophy and weakness observed in ICU patients with CIM were associated with the altered expression of genes involved in muscle contraction, newly identified E3 ligases, autophagy and calpain systems, apoptosis, and chaperone expression. In addition, MYOD1, p38 MAPK, and dexamethasone were identified as potential upstream regulators of skeletal muscle gene expression in ICU patients with CIM. Mechanical loading only marginally affected the skeletal muscle transcriptome profiling of ICU patients diagnosed with CIM.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0194-1) contains supplementary material, which is available to authorized users.

Collagen XXV promotes myoblast fusion during myogenic differentiation and muscle formation

Fusion of myoblasts into multinucleated myofibers is crucial for skeletal muscle development and regeneration. However, the mechanisms controlling this process remain to be determined. Here we identified the involvement of a new extracellular matrix protein in myoblast fusion. Collagen XXV is a transmembrane-type collagen highly transcribed during early myogenesis when primary myofibers form. Limb muscles of E12.5 and E14.5 Col25a1−/− embryos show a clear defect in the formation of multinucleated myofibers. In cell culture, the cleaved soluble extracellular domain of the collagen XXV is sufficient to promote the formation of highly multinucleated myofibers. Col25a1 is transiently expressed during myogenic differentiation and Col25a1 transcripts are down-regulated in multinucleated myofibers by a muscle-specific microRNA, miR-499. Altogether, these findings indicate that collagen XXV is required in vivo and in vitro for the fusion of myoblasts into myofibers and give further evidence that microRNAs participate to the regulation of this process.

New Human Chromosomal Sites with "Safe Harbor" Potential for Targeted Transgene Insertion

This study identified 35 new sites for targeted transgene insertion that have the potential to serve as new human genomic "safe harbor" sites (SHS). SHS potential for these 35 sites, located on 16 chromosomes, including both arms of the human X chromosome, and for the existing human SHS AAVS1, hROSA26, and CCR5 was assessed using eight different desirable, widely accepted criteria for SHS verifiable with human genomic data. Three representative newly identified sites on human chromosomes 2 and 4 were then experimentally validated by in vitro and in vivo cleavage-sensitivity tests, and analyzed for population-level and cell line-specific sequence variants that might confound site targeting. The highly ranked site on chromosome 4 (SHS231) was further characterized by targeted homology-dependent and -independent transgene insertion and expression in different human cell lines. The structure and fidelity of transgene insertions at this site were confirmed, together with analyses that demonstrated stable expression and function of transgene-encoded proteins, including fluorescent protein markers, selectable marker cassettes, and Cas9 protein variants. SHS-integrated transgene-encoded Cas9 proteins were shown to be capable of introducing a large (17 kb) gRNA-specified deletion in the PAX3/FOXO1 fusion oncogene in human rhabdomyosarcoma cells and as a Cas9-VPR fusion protein to upregulate expression of the muscle-specific transcription factor MYF5 in human rhabdomyosarcoma cells. An engineering "toolkit" was developed to enable easy use of the most extensively characterized of these new human sites, SHS231, located on the proximal long arm of chromosome 4. The target sites identified here have the potential to serve as additional human SHS to enable basic and clinical gene editing and genome-engineering applications.

H3K27me3 Depletion during Differentiation Promotes Myogenic Transcription in Porcine Satellite Cells

BACKGROUND

Porcine skeletal muscle satellite cells play important roles in myogenesis and muscle regeneration. Integrated analysis of transcriptome and histone modifications would reveal epigenomic roles in promoting myogenic differentiation in swine.

METHODS

Porcine satellite cells (PSCs) were isolated and in-vitro cultured from newborn piglets. RNA Sequencing (RNA-Seq) and Chromatin Immunoprecipitation Sequencing (ChIP-Seq) experiments were performed using proliferating cells and terminal myotubes in order to interrogate the transcriptomic profiles, as well as the distribution of histone markers-H3K4me3, H3K27me3, and H3K27ac-and RNA polymerase II.

RESULTS

The study identified 917 differentially expressed genes during cell differentiation. The landscape of epigenetic marks was displayed on a genome-wide scale, which had globally shrunken. H3K27me3 reinforcement participated in obstructing the transcription of proliferation-related genes, while its depletion was closely related to the up-regulation of myogenic genes. Furthermore, the degree of H3K27me3 modification was dramatically reduced by 50%, and 139 myogenic genes were upregulated to promote cell differentiation.

CONCLUSIONS

The depletion of H3K27me3 was shown to promote porcine satellite cell differentiation through upregulating the transcription level of myogenic genes. Our findings in this study provide new insights of the epigenomic mechanisms occurring during myogenic differentiation, and shed light on chromatin states and the dynamics underlying myogenesis.