Glutathione depletion impairs myogenic differentiation of murine skeletal muscle C2C12 cells through sustained NF-kappaB activation

Hesperetin but not ellagic acid increases myosin heavy chain expression and cell fusion in C2C12 myoblasts in the presence of oxidative stress

Introduction

Skeletal muscle regeneration is impaired in elderly. An oxidative stress-induced decrease in differentiation capacity of muscle satellite cells is a key factor in this process. The aim of this study is to investigate whether orange polyphenol hesperetin and pomegranate polyphenol ellagic acid enhance myoblast differentiation in the presence and absence of oxidative stress, and to explore underlying mechanisms.

Methods

C2C12 myoblasts were proliferated for 24 h and differentiated for 120 h while exposed to hesperetin (5, 20, 50 μM), ellagic acid (0.05, 0.1 μM) or a combination (20 μM hesperetin, 0.05 μM ellagic acid) with and without oxidative stress-inducing compound menadione (9 μM) during 24 h of proliferation and during the first 5 h of differentiation. The number of proliferating cells was assessed using fluorescent labeling of incorporated 5-ethynyl-2'-deoxyuridine. Myosin heavy chain expression was assessed by fluorescence microscopy and cell fusion index was calculated. Furthermore, protein expression of phosphorylated p38 and myomixer were assessed using Western blot.

Results

None of the compounds induced effects on cell proliferation. Without menadione, 50 μM hesperetin increased fusion index by 12.6% compared to control (p < 0.01), while ellagic acid did not affect measured parameters of differentiation. Menadione treatment did not change myosin heavy chain expression and fusion index. In combination with menadione, 20 μM hesperetin increased myosin heavy chain expression by 35% (p < 0.01) and fusion index by 7% (p = 0.04) compared to menadione. Furthermore, the combination of menadione with hesperetin and ellagic acid increased myosin heavy chain expression by 35% compared to menadione (p = 0.02). Hesperetin and ellagic acid did not change p38 phosphorylation and myomixer expression compared to control, while treatment with menadione increased p38 phosphorylation (p < 0.01) after 5 h and decreased myomixer expression (p = 0.04) after 72 h of differentiation.

Conclusion and discussion

Hesperetin increased myosin heavy chain expression in the presence of oxidative stress induced by menadione, and increased cell fusion both in the presence and absence of menadione. Ellagic acid did not affect the measured parameters of myoblast differentiation. Therefore, hesperetin should be considered as nutritional prevention or treatment strategy to maintain muscle function in age-related diseases such as sarcopenia. Future research should focus on underlying mechanisms and translation of these results to clinical practice.

Cystine/glutamate antiporter xCT controls skeletal muscle glutathione redox, bioenergetics and differentiation

Cysteine, the rate-controlling amino acid in cellular glutathione synthesis is imported as cystine, by the cystine/glutamate antiporter, xCT, and subsequently reduced to cysteine. As glutathione redox is important in muscle regeneration in aging, we hypothesized that xCT exerts upstream control over skeletal muscle glutathione redox, metabolism and regeneration. Bioinformatic analyses of publicly available datasets revealed that expression levels of xCT and GSH-related genes are inversely correlated with myogenic differentiation genes. Muscle satellite cells (MuSCs) isolated from Slc7a11sut/sut mice, which harbour a mutation in the Slc7a11 gene encoding xCT, required media supplementation with 2-mercaptoethanol to support cell proliferation but not myotube differentiation, despite persistently lower GSH. Slc7a11sut/sut primary myotubes were larger compared to WT myotubes, and also exhibited higher glucose uptake and cellular oxidative capacities. Immunostaining of myogenic markers (Pax7, MyoD, and myogenin) in cardiotoxin-damaged tibialis anterior muscle fibres revealed greater MuSC activation and commitment to differentiation in Slc7a11sut/sut muscle compared to WT mice, culminating in larger myofiber cross-sectional areas at 21 days post-injury. Slc7a11sut/sut mice subjected to a 5-week exercise training protocol demonstrated enhanced insulin tolerance compared to WT mice, but blunted muscle mitochondrial biogenesis and respiration in response to exercise training. Our results demonstrate that the absence of xCT inhibits cell proliferation but promotes myotube differentiation by regulating cellular metabolism and glutathione redox. Altogether, these results support the notion that myogenesis is a redox-regulated process and may help inform novel therapeutic approaches for muscle wasting and dysfunction in aging and disease.

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

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

p66Shc signaling and autophagy impact on C2C12 myoblast differentiation during senescence

During aging, muscle regenerative capacities decline, which is concomitant with the loss of satellite cells that enter in a state of irreversible senescence. However, what mechanisms are involved in myogenic senescence and differentiation are largely unknown. Here, we showed that early-passage or "young" C2C12 myoblasts activated the redox-sensitive p66Shc signaling pathway, exhibited a strong antioxidant protection and a bioenergetic profile relying predominantly on OXPHOS, responses that decrease progressively during differentiation. Furthermore, autophagy was increased in myotubes. Otherwise, late-passage or "senescent" myoblasts led to a highly metabolic profile, relying on both OXPHOS and glycolysis, that may be influenced by the loss of SQSTM1/p62 which tightly regulates the metabolic shift from aerobic glycolysis to OXPHOS. Furthermore, during differentiation of late-passage C2C12 cells, both p66Shc signaling and autophagy were impaired and this coincides with reduced myogenic capacity. Our findings recognized that the lack of p66Shc compromises the proliferation and the onset of the differentiation of C2C12 myoblasts. Moreover, the Atg7 silencing favored myoblasts growth, whereas interfered in the viability of differentiated myotubes. Then, our work demonstrates that the p66Shc signaling pathway, which highly influences cellular metabolic status and oxidative environment, is critical for the myogenic commitment and differentiation of C2C12 cells. Our findings also support that autophagy is essential for the metabolic switch observed during the differentiation of C2C12 myoblasts, confirming how its regulation determines cell fate. The regulatory roles of p66Shc and autophagy mechanisms on myogenesis require future attention as possible tools that could predict and measure the aging-related state of frailty and disability.

Roles of Glutathione and AP-1 in the Enhancement of Vitamin D-Induced Differentiation by Activators of the Nrf2 Signaling Pathway in Acute Myeloid Leukemia Cells

Active vitamin D derivatives (VDDs)-1α,25-dihydroxyvitamin D3/D2 and their synthetic analogs-are well-known inducers of cell maturation with the potential for differentiation therapy of acute myeloid leukemia (AML). However, their dose-limiting calcemic activity is a significant obstacle to using VDDs as an anticancer treatment. We have shown that different activators of the NF-E2-related factor-2/Antioxidant Response Element (Nrf2/ARE) signaling pathway, such as the phenolic antioxidant carnosic acid (CA) or the multiple sclerosis drug monomethyl fumarate (MMF), synergistically enhance the antileukemic effects of various VDDs applied at low concentrations in vitro and in vivo. This study aimed to investigate whether glutathione, the major cellular antioxidant and the product of the Nrf2/ARE pathway, can mediate the Nrf2-dependent differentiation-enhancing activity of CA and MMF in HL60 human AML cells. We report that glutathione depletion using L-buthionine sulfoximine attenuated the enhancing effects of both Nrf2 activators concomitant with downregulating vitamin D receptor (VDR) target genes and the activator protein-1 (AP-1) family protein c-Jun levels and phosphorylation. On the other hand, adding reduced glutathione ethyl ester to dominant negative Nrf2-expressing cells restored both the suppressed differentiation responses and the downregulated expression of VDR protein, VDR target genes, as well as c-Jun and P-c-Jun levels. Finally, using the transcription factor decoy strategy, we demonstrated that AP-1 is necessary for the enhancement by CA and MMF of 1α,25-dihydroxyvitamin D3-induced VDR and RXRα protein expression, transactivation of the vitamin D response element, and cell differentiation. Collectively, our findings suggest that glutathione mediates, at least in part, the potentiating effect of Nrf2 activators on VDDs-induced differentiation of AML cells, likely through the positive regulation of AP-1.

Effects of maternal inflammation on growth performance, carcass characteristics, and meat quality of offspring pigs

The objective of this research was to determine the effects of mid-gestational maternal inflammation on performance, carcass characteristics, and meat quality of offspring. Pregnant gilts were administered either lipopolysaccharide (LPS; n = 7) or saline (CON, n = 7) from days 70 to 84 of gestation. Gilts assigned to the LPS treatment were administered an intravenous injection of reconstituted LPS every other day with a beginning dose of 10 μg LPS/kg body weight and subsequent doses increasing by 12%, while CON gilts received intravenous injections of comparable volumes of saline. Gilts farrowed naturally, and at day 66 of age, a total of 59 pigs, both barrows and gilts began a 3-phase feeding regimen designed to meet or exceed nutrient requirements for growing-finishing pigs. Pigs were weighed on days 0, 35, 70, and 105 of the finishing trial to determine average daily gain, average daily feed intake, and gain-to-feed ratio (G:F). On day 106, pigs were slaughtered under the supervision of the U.S. Department of Agriculture Food Safety Inspection Service. Ending live weight, hot carcass weight, and dressing percentage were determined. The left side of carcasses was weighed and fabricated to determine carcass cutting yields. The semitendinosus was collected for histological samples. Fresh belly characteristics and loin quality were measured. Two chops were collected for Warner-Bratzler shear force and proximate analysis. No differences (P ≥ 0.13) between LPS and CON pigs were observed for growth performance in phases 1, 2, 3, or overall (days 0 to 105) performance with the exception of overall G:F reduced in CON pigs compared with LPS pigs (P = 0.03). There was a tendency for carcass yield to be reduced (P = 0.06; 0.82% units) in LPS pigs compared with CON pigs. Additionally, longissimus muscle area tended to be reduced (P = 0.10) 2.27 cm2 in LPS compared with CON pigs. Loin chop quality traits including instrumental color, subjective color, marbling, firmness, pH, and drip loss were not different (P ≥ 0.09) between LPS and CON pigs. Fresh belly characteristics were not different (P ≥ 0.22) between LPS and CON pigs. There were no differences in primal and subprimal weights, except that LPS pigs tended to have a reduction (P ≥ 0.07) in tenderloin and Canadian back weights compared with CON pigs. Furthermore, LPS pigs had no differences (P ≥ 0.25) in muscle fiber composition or size; however, LPS pigs tended (P = 0.10) to have a 13% reduction in estimated muscle fibers number compared with CON pigs. In summary, mid-gestational inflammation did not result in reduced meat quality, growth performance, or carcass yields of offspring.

β‑carotene attenuates muscle wasting in cancer cachexia by regulating myogenesis and muscle atrophy

Cancer cachexia is a metabolic disease involving multiple organs, which is accompanied by the depletion of muscle tissue and is associated with ~20% of cancer‑related deaths. Muscle wasting is a critical factor in cancer cachexia. β‑carotene (BC) has been shown to increase muscle mass and hypertrophy in healthy mice. However, its effects on muscle tissue dysregulation in cancer cachexia have yet to be studied. In the present study, 5‑week‑old male C57BL/6J mice were injected with 1x106 Lewis lung carcinoma (LLC) cells to induce cancer cachexia; then the mice were administered BC (4 or 8 mg/kg) for 22 days to assess its effects on muscle atrophy in the gastrocnemius muscles. The effects of BC on inflammatory cytokines, myogenesis and muscle atrophy were evaluated using C2C12 myotubes treated with LLC‑conditioned media. BC supplementation significantly suppressed tumor growth, inflammatory cytokines, and hepatic gluconeogenesis in the LLC‑induced cancer cachexia mouse model, while also improving muscle weight and grip strength. These effects are considered to be mediated by the PI3K/Akt pathway and through regulation of muscle atrophy. Moreover, BC treatment was associated with the recovery of LLC‑conditioned media‑induced muscle differentiation deficits and muscle atrophy in C2C12 myotubes. These findings indicate BC as a potential novel therapeutic agent for cancer cachexia.

Heme oxygenase-1: A potential therapeutic target for improving skeletal muscle atrophy

Skeletal muscle atrophy is a common muscle disease that is directly caused by an imbalance in protein synthesis and degradation. At the histological level, it is mainly characterized by a reduction in muscle mass and fiber cross-sectional area (CSA). Patients with skeletal muscle atrophy present with reduced motor ability, easy fatigue, and poor life quality. Heme oxygenase-1 (HO-1) is an inducible enzyme that catalyzes the degradation of heme and has attracted much attention for its anti-oxidation effects. In addition, there is growing evidence that HO-1 plays an important role in anti-inflammatory, anti-apoptosis, pro-angiogenesis, and maintaining skeletal muscle homeostasis, making it a potential therapeutic target for improving skeletal muscle atrophy. Here, we review the pathogenesis of skeletal muscle atrophy, the biology of HO-1 and its regulation, and the biological function of HO-1 in skeletal muscle homeostasis, with a specific focus on the role of HO-1 in skeletal muscle atrophy, aiming to observe the therapeutic potential of HO-1 for skeletal muscle atrophy.

Transcriptomic Response of Differentiating Porcine Myotubes to Thermal Stress and Donor Piglet Age

Climate change is a current concern that directly and indirectly affects agriculture, especially the livestock sector. Neonatal piglets have a limited thermoregulatory capacity and are particularly stressed by ambient temperatures outside their optimal physiological range, which has a major impact on their survival rate. In this study, we focused on the effects of thermal stress (35 °C, 39 °C, and 41 °C compared to 37 °C) on differentiating myotubes derived from the satellite cells of Musculus rhomboideus, isolated from two different developmental stages of thermolabile 5-day-old (p5) and thermostable 20-day-old piglets (p20). Analysis revealed statistically significant differential expression genes (DEGs) between the different cultivation temperatures, with a higher number of genes responding to cold treatment. These DEGs were involved in the macromolecule degradation and actin kinase cytoskeleton categories and were observed at lower temperatures (35 °C), whereas at higher temperatures (39 °C and 41 °C), the protein transport system, endoplasmic reticulum system, and ATP activity were more pronounced. Gene expression profiling of HSP and RBM gene families, which are commonly associated with cold and heat responses, exhibited a pattern dependent on temperature variability. Moreover, thermal stress exhibited an inhibitory effect on cell cycle, with a more pronounced downregulation during cold stress driven by ADGR genes. Additionally, our analysis revealed DEGs from donors with an undeveloped thermoregulation capacity (p5) and those with a fully developed thermoregulation capacity (p20) under various cultivation temperature. The highest number of DEGs and significant GO terms was observed under temperatures of 35 °C and 37 °C. In particular, under 35 °C, the DEGs were enriched in insulin, thyroid hormone, and calcium signaling pathways. This result suggests that the different thermoregulatory capacities of the donor piglets determined the ability of the primary muscle cell culture to differentiate into myotubes at different temperatures. This work sheds new light on the underlying molecular mechanisms that govern piglet differentiating myotube response to thermal stress and can be leveraged to develop effective thermal management strategies to enhance skeletal muscle growth.

Interplay between Protein Kinase C Epsilon and Reactive Oxygen Species during Myogenic Differentiation

Reactive oxygen species (ROS) are currently recognized as a key driver of several physiological processes. Increasing evidence indicates that ROS levels can affect myogenic differentiation, but the molecular mechanisms still need to be elucidated. Protein kinase C (PKC) epsilon (PKCe) promotes muscle stem cell differentiation and regeneration of skeletal muscle after injury. PKCs play a tissue-specific role in redox biology, with specific isoforms being both a target of ROS and an up-stream regulator of ROS production. Therefore, we hypothesized that PKCe represents a molecular link between redox homeostasis and myogenic differentiation. We used an in vitro model of a mouse myoblast cell line (C2C12) to study the PKC-redox axis. We demonstrated that the transition from a myoblast to myotube is typified by increased PKCe protein content and decreased ROS. Intriguingly, the expression of the antioxidant enzyme superoxide dismutase 2 (SOD2) is significantly higher in the late phases of myogenic differentiation, mimicking PKCe protein content. Furthermore, we demonstrated that PKCe inhibition increases ROS and reduces SOD2 protein content while SOD2 silencing did not affect PKCe protein content, suggesting that the kinase could be an up-stream regulator of SOD2. To support this hypothesis, we found that in C2C12 cells, PKCe interacts with Nrf2, whose activation induces SOD2 transcription. Overall, our results indicate that PKCe is capable of activating the antioxidant signaling preventing ROS accumulation in a myotube, eventually promoting myogenic differentiation.

Characterization of Glutathione Dithiophosphates as Long-Acting H2S Donors

Considering the important cytoprotective and signaling roles but relatively narrow therapeutic index of hydrogen sulfide (H₂S), advanced H₂S donors are required to achieve a therapeutic effect. In this study, we proposed glutathione dithiophosphates as new combination donors of H₂S and glutathione. The kinetics of H₂S formation in dithiophosphate solutions suggested a continuous H₂S release by the donors, which was higher for the dithiophosphate of reduced glutathione than oxidized glutathione. The compounds, unlike NaHS, inhibited the proliferation of C2C12 myoblasts at submillimolar concentrations due to an efficient increase in intracellular H₂S. The H₂S donors more profoundly affected reactive oxygen species and reduced glutathione levels in C2C12 myocytes, in which these parameters were elevated compared to myoblasts. Oxidized glutathione dithiophosphate as well as control donors exerted antioxidant action toward myocytes, whereas the effect of reduced glutathione dithiophosphate at (sub-)micromolar concentrations was rather modulating. This dithiophosphate showed an enhanced negative inotropic effect mediated by H₂S upon contraction of the atrial myocardium, furthermore, its activity was prolonged and reluctant for washing. These findings identify glutathione dithiophosphates as redox-modulating H₂S donors with long-acting profile, which are of interest for further pharmacological investigation.

Effect of RONS-Induced Intracellular Redox Homeostasis in 6-NBDG/Glucose Uptake in C2C12 Myotubes and Single Isolated Skeletal Muscle Fibres

The glucose uptake in skeletal muscle is essential to produce energy through ATP, which is needed by this organ to maintain vital functions. The impairment of glucose uptake compromises the metabolism and function of skeletal muscle and other organs and is a feature of diabetes, obesity, and ageing. There is a need for research to uncover the mechanisms involved in the impairment of glucose uptake in skeletal muscle. In this study, we adapted, developed, optimised, and validated a methodology based on the fluorescence glucose analogue 6-NBDG, combined with a quantitative fluorescence microscopy image analysis, to determine the glucose uptake in two models of skeletal muscle cells: C2C12 myotubes and single fibres isolated from muscle. It was proposed that reactive oxygen and nitrogen species (RONS) and redox homeostasis play an important role in the modulation of intracellular redox signalling pathways associated with glucose uptake. In this study, we prove that the prooxidative intracellular redox environment under oxidative eustress produced by RONS such as hydrogen peroxide and nitric oxide improves glucose uptake in skeletal muscle cells. However, when oxidation is excessive, oxidative distress occurs, and cellular viability is compromised, although there might be an increase in the glucose uptake. Based on the results of this study, the determination of 6-NBDG/glucose uptake in myotubes and skeletal muscle cells is feasible, validated, and will contribute to improve future research.

Metallothionein Gene Deficiency Facilitates the Differentiation of C2C12 Myoblasts into Slow-Twitch Myotubes

Metallothionein (MT) 1 and 2 are ubiquitously expressed cysteine-rich, low molecular weight proteins. MT expression is upregulated in skeletal muscle during aging. MTs also play role in multiple types of skeletal muscle atrophy. Meanwhile, it has been reported that MT1 and MT2 gene deficiency increases myogenesis in MT knockout (MTKO) mice. However, little is known about the effect of MTs on muscle formation and atrophy. In this study, we investigated the effect of MT1 and MT2 gene knock-out using the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (CRISPR-Cas9) system in an in vitro skeletal muscle differentiation model (C2C12 cell line). MT deficiency promoted myogenic differentiation and myotube formation in C2C12 cells. Muscle-specific transcription factors MyoD and myogenin were found to be upregulated at the late stage of myotube differentiation in MTKO cells. Furthermore, the fast-twitch myosin heavy chain (MyHC) protein expression was similar in MTKO and mock-transfected myotubes, but slow-MyHC expression was higher in MTKO cells than in mock cells. The MT gene deletion did not affect the number of fast MyHC-positive myotubes but increased the number of slow MyHC-positive myotubes. Treatment with the antioxidant N-acetylcysteine (NAC) inhibited the increase in the number of slow MyHC-positive myotubes as well as slow-MyHC expression in MTKO cells. In contrast, NAC treatment did not alter the number of fast MyHC-positive myotubes or the expression of fast-MyHC in MTKO cells. These results suggest that the antioxidant effects of MTs may be involved in slow-twitch myofiber formation in skeletal muscle.

Functional Nutrients to Ameliorate Neurogenic Muscle Atrophy

Neurogenic muscle atrophy is a debilitating condition that occurs from nerve trauma in association with diseases or during aging, leading to reduced interaction between motoneurons and skeletal fibers. Current therapeutic approaches aiming at preserving muscle mass in a scenario of decreased nervous input include physical activity and employment of drugs that slow down the progression of the condition yet provide no concrete resolution. Nutritional support appears as a precious tool, adding to the success of personalized medicine, and could thus play a relevant part in mitigating neurogenic muscle atrophy. We herein summarize the molecular pathways triggered by denervation of the skeletal muscle that could be affected by functional nutrients. In this narrative review, we examine and discuss studies pertaining to the use of functional ingredients to counteract neurogenic muscle atrophy, focusing on their preventive or curative means of action within the skeletal muscle. We reviewed experimental models of denervation in rodents and in amyotrophic lateral sclerosis, as well as that caused by aging, considering the knowledge generated with use of animal experimental models and, also, from human studies.

NRF2 Regulates Viability, Proliferation, Resistance to Oxidative Stress, and Differentiation of Murine Myoblasts and Muscle Satellite Cells

Increased oxidative stress can slow down the regeneration of skeletal muscle and affect the activity of muscle satellite cells (mSCs). Therefore, we evaluated the role of the NRF2 transcription factor (encoded by the Nfe2l2 gene), the main regulator of the antioxidant response, in muscle cell biology. We used (i) an immortalized murine myoblast cell line (C2C12) with stable overexpression of NRF2 and (ii) primary mSCs isolated from wild-type and Nfe2l2 (transcriptionally)-deficient mice (Nfe2l2^tKO). NRF2 promoted myoblast proliferation and viability under oxidative stress conditions and decreased the production of reactive oxygen species. Furthermore, NRF2 overexpression inhibited C2C12 cell differentiation by down-regulating the expression of myogenic regulatory factors (MRFs) and muscle-specific microRNAs. We also showed that NRF2 is indispensable for the viability of mSCs since the lack of its transcriptional activity caused high mortality of cells cultured in vitro under normoxic conditions. Concomitantly, Nfe2l2^tKO mSCs grown and differentiated under hypoxic conditions were viable and much more differentiated compared to cells isolated from wild-type mice. Taken together, NRF2 significantly influences the properties of myoblasts and muscle satellite cells. This effect might be modulated by the muscle microenvironment.

Porous Se@SiO2 nanoparticles improve oxidative injury to promote muscle regeneration via modulating mitochondria

Background: Acute skeletal muscle injuries are common among physical or sports traumas. The excessive oxidative stress at the site of injury impairs muscle regeneration. The authors have recently developed porous Se@SiO₂ nanoparticles (NPs) with antioxidant properties. Methods: The protective effects were evaluated by cell proliferation, myogenic differentiation and mitochondrial activity. Then, the therapeutic effect was investigated in a cardiotoxin-induced muscle injury rat model. Results: Porous Se@SiO₂ NPs significantly protected the morphological and functional stability of mitochondria, thus protecting satellite cells from H₂O₂-induced damage to cell proliferation and myogenic differentiation. In the rat model, intervention with porous Se@SiO₂ NPs promoted muscle regeneration. Conclusion: This study reveals the application potential of porous Se@SiO₂ NPs in skeletal muscle diseases related to mitochondrial dysfunction.

The mitochondrial adenine nucleotide transporters in myogenesis

Adenine Nucleotide Translocator isoforms (ANTs) exchange ADP/ATP across the inner mitochondrial membrane, are also voltage-activated proton channels and regulate mitophagy and apoptosis. The ANT1 isoform predominates in heart and muscle while ANT2 is systemic. Here, we report the creation of Ant mutant mouse myoblast cell lines with normal Ant1 and Ant2 genes, deficient in either Ant1 or Ant2, and deficient in both the Ant1 and Ant2 genes. These cell lines are immortal under permissive conditions (IFN-γ + serum at 32 °C) permitting expansion but return to normal myoblasts that can be differentiated into myotubes at 37 °C. With this system we were able to complement our Ant1 mutant studies by demonstrating that ANT2 is important for myoblast to myotube differentiation and myotube mitochondrial respiration. ANT2 is also important in the regulation of mitochondrial biogenesis and antioxidant defenses. ANT2 is also associated with increased oxidative stress response and modulation for Ca⁺⁺ sequestration and activation of the mitochondrial permeability transition (mtPTP) pore during cell differentiation.

Redox Signaling and Stress in Inherited Myopathies

Significance: Reactive oxygen species (ROS) are highly reactive compounds that behave like a double-edged sword; they damage cellular structures and act as second messengers in signal transduction. Mitochondria and endoplasmic reticulum (ER) are interconnected organelles with a central role in ROS production, detoxification, and oxidative stress response. Skeletal muscle is the most abundant tissue in mammals and one of the most metabolically active ones and thus relies mainly on oxidative phosphorylation (OxPhos) to synthesize adenosine triphosphate. The impairment of OxPhos leads to myopathy and increased ROS production, thus affecting both redox poise and signaling. In addition, ROS enter the ER and trigger ER stress and its maladaptive response, which also lead to a myopathic phenotype with mitochondrial involvement. Here, we review the role of ROS signaling in myopathies due to either mitochondrial or ER dysfunction. Recent Advances: Relevant advances have been evolving over the last 10 years on the intricate ROS-dependent pathways that act as modifiers of the disease course in several myopathies. To this end, pathways related to mitochondrial biogenesis, satellite cell differentiation, and ER stress have been studied extensively in myopathies. Critical Issues: The analysis of the chemistry and the exact quantitation, as well as the localization of ROS, are still challenging due to the intrinsic labile nature of ROS and the technical limitations of their sensors. Future Directions: The mechanistic studies of the pathogenesis of mitochondrial and ER-related myopathies offer a unique possibility to discover novel ROS-dependent pathways. Antioxid. Redox Signal. 37, 301-323.

Extracellular polysaccharides purified (Polycan) from Aureobasidium pullulans SM‑2001 improves pathophysiology of dystrophin-deficient mdx mice

Background

Duchenne muscular dystrophy is a hereditary muscular disease involving degeneration (i.e. atrophy and loss of muscle fibres) of skeletal muscles, including the diaphragm, and progressively severe functional decline. A previous study shows Polycan, a type of β-glucan derived from the black yeast Aureobasidium pullulans (SM-2001), promotes osteogenicity and bone loss, and possesses anti-inflammatory activity to induce inflammatory cytokines in human immune and cancer cells.

Objective

In this study, we evaluated changes in exercise load behaviour measurements and changes in muscle-related physiological indicators following oral administration of Polycan in mdx mice, an experimental animal model of Duchenne muscular dystrophy.

Result

In mdx mice, Polycan prevented weight loss and thickness of skeletal muscle. In addition, by monitoring increases in running time of mice on treadmills and performing a grip strength test, we confirmed reduced muscle function was recovered to some extent after administering Polycan to mdx mice. In addition, we confirmed that Polycan significantly altered mRNA expression in a concentration-dependent manner, whereby myogenic transcription factors (MyoD, Myf5 and Myogenin) increased and FoxO3α, MuRF1 and Atrogin-1 decreased. We aimed to investigate the mechanism of action in Polycan on energy metabolism of p-AMPK, SIRT1 and PGC1α with apoptosis expression levels as factors related to signalling pathways. Expression ratios of cleaved-caspase-3/caspase-3 and Bax/Bcl-2 in the Polycan extract-administered group increased compared with the control group.

Conclusion

These results demonstrate that Polycan can improve and protect muscle atrophy by preventing apoptosis via pathway regulation related to myogenic transcription factors and energy metabolism in mdx mice.

Effects of dietary tryptophan on muscle growth, protein synthesis and antioxidant capacity in hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂

The present study evaluated effects of dietary supplementation with tryptophan (Trp) on muscle growth, protein synthesis and antioxidant capacity in hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. Fish were fed six different diets containing 2·6 (control), 3·1, 3·7, 4·2, 4·7 and 5·6 g Trp/kg diet for 56 d, respectively. Results showed that dietary Trp significantly (1) improved muscle protein content, fibre density and frequency of fibre diameter; (2) up-regulated the mRNA levels of PCNA, myf5, MyoD1, MyoG, MRF4, IGF-I, IGF-II, IGF-IR, PIK3Ca, TOR, 4EBP1 and S6K1; (3) increased phosphorylation levels of AKT, TOR and S6K1; (4) decreased contents of MDA and PC, and increased activities of CAT, GST, GR, ASA and AHR; (5) up-regulated mRNA levels of CuZnSOD, CAT, GST, GPx, GCLC and Nrf2, and decreased Keap1 mRNA level; (6) increased nuclear Nrf2 protein level and the intranuclear antioxidant response element-binding ability, and reduced Keap1 protein level. These results indicated that dietary Trp improved muscle growth, protein synthesis as well as antioxidant capacity, which might be partly related to myogenic regulatory factors, IGF/PIK3Ca/AKT/TOR and Keap1/Nrf2 signalling pathways. Finally, based on the quadratic regression analysis of muscle protein and MDA contents, the optimal Trp requirements of hybrid catfish (21·82-39·64 g) were estimated to be 3·94 and 3·93 g Trp/kg diet (9·57 and 9·54 g/kg of dietary protein), respectively.

Understanding Myoblast Differentiation Pathways When Cultured on Electroactive Scaffolds through Proteomic Analysis

Electroactive materials allow the modulation of cell-materials interactions and cell fate, leading to advanced tissue regeneration strategies. Nevertheless, their effect at the cellular level is still poorly understood. In this context, the proteome analysis of C2C12 cell differentiation cultured on piezoelectric polymer films with null average surface charge (non-poled), net positive surface charge (poled +), and net negative surface charge (poled -) has been addressed. Protein/pathway alterations for skeletal muscle development were identified comparing proteomic profiles of C2C12 cells differentiated on poly(vinylidene fluoride), with similar cells differentiated on a polystyrene plate (control), using label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS). Only significantly expressed proteins (P < 0.01, analysis of variance) were used for bioinformatic analyses. A total of 37 significantly expressed proteins were detected on the C2C12 proteome with PVDF "poled -" at 24 h, whereas on the PVDF "poled +", a total of 105 significantly expressed proteins were considered. At 5 days of differentiation, the number of significantly expressed proteins decreased to 23 and 31 in cells grown on negative and positive surface charge, respectively, the influence of surface charge being more explicit in some proteins. In both cases, proteins such as Fbn1, Hspg2, Rcn3, Tgm2, Mylpf, Anxa2, and Anxa6, involved in calcium-related signaling, were highly expressed during myoblast differentiation. Furthermore, some proteins involved in muscle contraction (Acta2, Anxa2, and Anxa6) were detected in the PVDF "poled +" sample. Upregulation of several proteins that enhance skeletal muscle development was detected in the PVDF "poled -" sample, including Ckm (422%), Tmem14c (384%), Serpinb6a (460%), adh7 (199%), and Car3 (171%), while for the "poled +" samples, these proteins were also upregulated at a smaller magnitude (254, 317, 253, 123, and 72%, respectively). Other differentially expressed proteins such as Mylpf (189%), Mybph (168%), and Mbnl1 (168%) were upregulated only in PVDF "poled -" samples, while Hba-a1 levels (581%) were increased in the PVDF "poled +" sample. On the other hand, cells cultured on non-poled samples have no differences with respect to the ones cultured on the control, in contrary to the poled films, with overall surface charge, demonstrating the relevance of scaffold surface charge on cell behavior. This study demonstrates that both positive and negative overall surface charges promote the differentiation of C2C12 cells through involvement of proteins related with the contraction of the skeletal muscle cells, with a more pronounced effect with the negative charged surfaces.

Regulation of A-to-I RNA editing and stop codon recoding to control selenoprotein expression during skeletal myogenesis

Selenoprotein N (SELENON), a selenocysteine (Sec)-containing protein with high reductive activity, maintains redox homeostasis, thereby contributing to skeletal muscle differentiation and function. Loss-of-function mutations in SELENON cause severe neuromuscular disorders. In the early-to-middle stage of myoblast differentiation, SELENON maintains redox homeostasis and modulates endoplasmic reticulum (ER) Ca2+ concentration, resulting in a gradual reduction from the middle-to-late stages due to unknown mechanisms. The present study describes post-transcriptional mechanisms that regulate SELENON expression during myoblast differentiation. Part of an Alu element in the second intron of SELENON pre-mRNA is frequently exonized during splicing, resulting in an aberrant mRNA that is degraded by nonsense-mediated mRNA decay (NMD). In the middle stage of myoblast differentiation, ADAR1-mediated A-to-I RNA editing occurs in the U1 snRNA binding site at 5' splice site, preventing Alu exonization and producing mature mRNA. In the middle-to-late stage of myoblast differentiation, the level of Sec-charged tRNASec decreases due to downregulation of essential recoding factors for Sec insertion, thereby generating a premature termination codon in SELENON mRNA, which is targeted by NMD.

Multilayered omics reveal sex- and depot-dependent adipose progenitor cell heterogeneity

Single-cell RNA sequencing (scRNA-seq) has revealed that adult white adipose tissue (WAT) harbors functionally diverse subpopulations of mesenchymal stromal cells that differentially impact tissue plasticity. To date, the molecular basis of this cellular heterogeneity has not been fully defined. Here, we describe a multilayered omics approach to dissect adipose progenitor cell heterogeneity in three dimensions: progenitor subpopulation, sex, and anatomical localization. We applied state-of-the-art mass spectrometry methods to quantify 4,870 proteins in eight different stromal cell populations from perigonadal and inguinal WAT of male and female mice and acquired transcript expression levels of 15,477 genes using RNA-seq. Our data unveil molecular signatures defining sex differences in preadipocyte differentiation and identify regulatory pathways that functionally distinguish adipose progenitor subpopulations. This multilayered omics analysis, freely accessible at http://preadprofiler.net/, provides unprecedented insights into adipose stromal cell heterogeneity and highlights the benefit of complementary proteomics to support findings from scRNA-seq studies.

The Role of Oxidative Stress in Skeletal Muscle Myogenesis and Muscle Disease

The contractile activity, high oxygen consumption and metabolic rate of skeletal muscle cause it to continuously produce moderate levels of oxidant species, such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Under normal physiological conditions, there is a dynamic balance between the production and elimination of ROS/RNS. However, when the oxidation products exceed the antioxidant defense capacity, the body enters a state of oxidative stress. Myogenesis is an important process to maintain muscle homeostasis and the physiological function of skeletal muscle. Accumulating evidence suggests that oxidative stress plays a key role in myogenesis and skeletal muscle physiology and pathology. In this review, we summarize the sources of reactive oxygen species in skeletal muscle and the causes of oxidative stress and analyze the key role of oxidative stress in myogenesis. Then, we discuss the relationship between oxidative stress and muscle homeostasis and physiopathology. This work systematically summarizes the role of oxidative stress in myogenesis and muscle diseases and provides targets for subsequent antioxidant therapy and repair of inflammatory damage in noninflammatory muscle diseases.

SOD1 in ALS: Taking Stock in Pathogenic Mechanisms and the Role of Glial and Muscle Cells

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the loss of motor neurons in the brain and spinal cord. While the exact causes of ALS are still unclear, the discovery that familial cases of ALS are related to mutations in the Cu/Zn superoxide dismutase (SOD1), a key antioxidant enzyme protecting cells from the deleterious effects of superoxide radicals, suggested that alterations in SOD1 functionality and/or aberrant SOD1 aggregation strongly contribute to ALS pathogenesis. A new scenario was opened in which, thanks to the generation of SOD1 related models, different mechanisms crucial for ALS progression were identified. These include excitotoxicity, oxidative stress, mitochondrial dysfunctions, and non-cell autonomous toxicity, also implicating altered Ca2+ metabolism. While most of the literature considers motor neurons as primary target of SOD1-mediated effects, here we mainly discuss the effects of SOD1 mutations in non-neuronal cells, such as glial and skeletal muscle cells, in ALS. Attention is given to the altered redox balance and Ca2+ homeostasis, two processes that are strictly related with each other. We also provide original data obtained in primary myocytes derived from hSOD1(G93A) transgenic mice, showing perturbed expression of Ca2+ transporters that may be responsible for altered mitochondrial Ca2+ fluxes. ALS-related SOD1 mutants are also responsible for early alterations of fundamental biological processes in skeletal myocytes that may impinge on skeletal muscle functions and the cross-talk between muscle cells and motor neurons during disease progression.

Multi-functional SiO32--releasing hydrogel with bioinspired mechanical properties and biodegradability for vascularized skeletal muscle regeneration

Vascularized skeletal muscle regeneration remains a great medical need but significant challenge. Biomaterial strategies that can facilitate the regeneration of muscle fibers and blood vessels are unavailable. Herein, we report...

Magnesium Influences Membrane Fusion during Myogenesis by Modulating Oxidative Stress in C2C12 Myoblasts

Magnesium (Mg) is essential to skeletal muscle where it plays a key role in myofiber relaxation. Although the importance of Mg in the mature skeletal muscle is well established, little is known about the role of Mg in myogenesis. We studied the effects of low and high extracellular Mg in C2C12 myogenic differentiation. Non-physiological Mg concentrations induce oxidative stress in myoblasts. The increase of reactive oxygen species, which occurs during the early phase of the differentiation process, inhibits myoblast membrane fusion, thus impairing myogenesis. Therefore, correct Mg homeostasis, also maintained through a correct dietary intake, is essential to assure the regenerative capacity of skeletal muscle fibers.

Platelet releasate normalises the compromised muscle regeneration in a mouse model of hyperlipidaemia

NEW FINDINGS

What is the central question of this study? What is the impact of obesity-independent hyperlipidaemia on skeletal muscle stem cell function of ApoE-deficient (ApoE^-/- ) mice? What is the main finding and its importance? Compromised muscle stem cell function accounts for the impaired muscle regeneration in hyperlipidaemic ApoE^-/- mice. Importantly, impaired muscle regeneration is normalised by administration of platelet releasate.

ABSTRACT

Muscle satellite cells are important stem cells for skeletal muscle regeneration and repair after injury. ApoE-deficient mice, an established mouse model of hyperlipidaemia and atherosclerosis, show evidence of oxidative stress-induced lesions and fat infiltration in skeletal muscle followed by impaired repair after injury. However, the mechanisms underpinning attenuated muscle regeneration remain to be fully defined. Key to addressing the latter is to understand the properties of muscle stem cells from ApoE-deficient mice and their myogenic potential. Muscle stem cells from ApoE-deficient mice were cultured both ex vivo (on single fibres) and in vitro (primary myoblasts) and their myogenic capacity was determined. Skeletal muscle regeneration was studied on days 5 and 10 after cardiotoxin injury. ApoE-deficient muscle stem cells showed delayed activation and differentiation on single muscle fibres ex vivo. Impaired proliferation and differentiation profiles were also evident on isolated primary muscle stem cells in culture. ApoE-deficient mice displayed impaired skeletal muscle regeneration after acute injury in vivo. Administration of platelet releasate in ApoE-deficient mice reversed the deficits of muscle regeneration after acute injury to wild-type levels. These findings indicate that muscle stem cell myogenic potential is perturbed in skeletal muscle of a mouse model of hyperlipidaemia. We propose that platelet releasate could be a therapeutic intervention for conditions with associated myopathy such as peripheral arterial disease.

Redox-dependent regulation of satellite cells following aseptic muscle trauma: Implications for sports performance and nutrition

Skeletal muscle satellite cells (SCs) are indispensable for tissue regeneration, remodeling and growth. Following myotrauma, SCs are activated, and assist in tissue repair. Exercise-induced muscle damage (EIMD) is characterized by a pronounced inflammatory response and the production of reactive oxygen species (ROS). Experimental evidence suggests that SCs kinetics (the propagation from a quiescent to an activated/proliferative state) following EIMD is redox-dependent and interconnected with changes in the SCs microenvironment (niche). Animal studies have shown that following aseptic myotrauma, antioxidant and/or anti-inflammatory supplementation leads to an improved recovery and skeletal muscle regeneration through enhanced SCs kinetics, suggesting a redox-dependent molecular mechanism. Although evidence suggests that antioxidant/anti-inflammatory compounds may prevent performance deterioration and enhance recovery, there is lack of information regarding the redox-dependent regulation of SCs responses following EIMD in humans. In this review, SCs kinetics following aseptic myotrauma, as well as the intrinsic redox-sensitive molecular mechanisms responsible for SCs responses are discussed. The role of redox status on SCs function should be further investigated in the future with human clinical trials in an attempt to elucidate the molecular pathways responsible for muscle recovery and provide information for potential nutritional strategies aiming at performance recovery.

l-Cysteine and Vitamin D Co-Supplementation Alleviates Markers of Musculoskeletal Disorders in Vitamin D-Deficient High-Fat Diet-Fed Mice

Vitamin D (VD) deficiency is associated with musculoskeletal disorders. This study examines whether co-supplementation of l-cysteine (LC) and VD is better than monotherapy with LC or VD at alleviating musculoskeletal dyshomeostasis in the skeletal muscle of VD-deficient high-fat diet (HFD-VD-) fed mice. Mice were fed a healthy diet or an HFD; for VD-deficient animals, the mice were maintained on a HFD-VD-diet (16 weeks); after the first 8 weeks, the HFD-VD-diet-fed mice were supplemented for another 8 weeks with LC, VD-alone, or the same doses of LC + VD by oral gavage. Saline and olive oil served as controls. Myotubes were exposed with high-glucose, palmitate, Monocyte Chemoattractant Protein 1 (MCP-1), and Tumor Necrosis Factor (TNF), to mimic the in vivo microenvironment. In vitro deficiencies of glutathione and hydrogen sulfide were induced by knockdown of GCLC and CSE genes. Relative gene expression of biomarkers (myogenic: MyoD, Mef2c, Csrp3; muscle dystrophy: Atrogin1, Murf1, and Myostatin; bone modeling and remodeling: RANK, RANKL, OPG) were analyzed using qRT-PCR. Co-supplementatoin with LC + VD showed beneficial effects on gene expression of myogenic markers and OPG but reduced markers of dystrophy, RANK/RANKL in comparison to LC or VD alone-supplementation. In vitro myotubes treated with glutathione (GSH) precursors also showed a positive effect on OPG and the myogenesis genes, and inhibited RANK/RANKL and muscle-dystrophy markers. This study reveals that the co-supplementation of LC with VD significantly alleviates the markers of musculoskeletal disorders in the skeletal muscle better than monotherapy with LC or VD in HFD-VD-fed mice.

The calmodulin redox sensor controls myogenesis

Muscle aging is accompanied by blunted muscle regeneration in response to injury and disuse. Oxidative stress likely underlies this diminished response, but muscle redox sensors that act in regeneration have not yet been characterized. Calmodulin contains multiple redox sensitive methionines whose oxidation alters the regulation of numerous cellular targets. We have used the CRISPR-Cas9 system to introduce a single amino acid substitution M109Q that mimics oxidation of methionine to methionine sulfoxide in one or both alleles of the CALM1 gene, one of three genes encoding the muscle regulatory protein calmodulin, in C2C12 mouse myoblasts. When signaled to undergo myogenesis, mutated myoblasts failed to differentiate into myotubes. Although early myogenic regulatory factors were present, cells with the CALM1 M109Q mutation in one or both alleles were unable to withdraw from the cell cycle and failed to express late myogenic factors. We have shown that a single oxidative modification to a redox-sensitive muscle regulatory protein can halt myogenesis, suggesting a molecular target for mitigating the impact of oxidative stress in age-related muscle degeneration.

Effects of passive heating intervention on muscle hypertrophy and neuromuscular function: A preliminary systematic review with meta-analysis

Passive heating has been therapeutically used to treat a range of health conditions. Further, this intervention presents as a potential exercise mimetic strategy showing acute and chronic effects on skeletal muscle adaptation and neuromuscular systems. This systematic review and meta-analysis aimed to synthesise the existing evidence on the effects of passive heating on muscle hypertrophy and neuromuscular function. Seven databases were searched (i.e., PubMed, Web of Science, Scopus, CINAHL, EMBASE, Cochrane, and SPORTDiscus) from 1937 to October 2019. Eligible studies included original papers using healthy animals or human samples (≥18 years; both sexes) that have used a control group or condition. Ten original articles were included in this review and four in the meta-analysis. The meta-analysis detected an increase in muscle mass in animal samples seven days after passive heating (I² = 65%, P < 0.01). The systematic review showed preliminary evidence that repeated passive heating exposures may promote muscle hypertrophy in animals and humans. Moreover, augmented muscle strength (involuntary and voluntary) may be observed after long-term passive heating (animals and humans) and increases in corticospinal excitability in humans after a single passive heating session. Passive heating has shown some potential benefits for skeletal muscle mass gain and muscle force improvement. Therefore, it is plausible to suggest that passive heating might be a worthwhile alternative to be recommended as an exercise mimetic for those people who lack or are unable to complete sufficient exercise.

Mitochondrial Function in Muscle Stem Cell Fates

Mitochondria are crucial organelles that control cellular metabolism through an integrated mechanism of energy generation via oxidative phosphorylation. Apart from this canonical role, it is also integral for ROS production, fatty acid metabolism and epigenetic remodeling. Recently, a role for the mitochondria in effecting stem cell fate decisions has gained considerable interest. This is important for skeletal muscle, which exhibits a remarkable property for regeneration following injury, owing to satellite cells (SCs), the adult myogenic stem cells. Mitochondrial function is associated with maintaining and dictating SC fates, linked to metabolic programming during quiescence, activation, self-renewal, proliferation and differentiation. Notably, mitochondrial adaptation might take place to alter SC fates and function in the presence of different environmental cues. This review dissects the contribution of mitochondria to SC operational outcomes, focusing on how their content, function, dynamics and adaptability work to influence SC fate decisions.

Type I collagen promotes the migration and myogenic differentiation of C2C12 myoblastsviathe release of interleukin-6 mediated by FAK/NF-κB p65 activation

Type I collagen has the potential to promote the migration and differentiation of C₂C₁₂myoblastviaIL-6 release that was mediated by FAK/NF-κB pathway.

Effect of dietary threonine on growth performance and muscle growth, protein synthesis and antioxidant-related signalling pathways of hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂

The experiment was conducted to investigate the effects of dietary threonine (Thr) on growth performance and muscle growth, protein synthesis and antioxidant-related signalling pathways of hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. A total of 1200 fish (14·19 (se 0·13) g) were randomly distributed into six groups with four replicates each, fed six diets with graded level of Thr (9·5, 11·5, 13·5, 15·4, 17·4 and 19·3 g/kg diets) for 56 d. Results showed (P < 0·05) that dietary Thr (1) increased percentage weight gain, specific growth rate, feed efficiency and protein efficiency ratio; (2) up-regulated growth hormone (GH), insulin-like growth factor 1 (IGF-1), proliferating cell nuclear antigen, myogenic regulation factors (MyoD, Myf5, MyoG and Mrf4) and myosin heavy chain (MyHC) mRNA levels; (3) increased muscle protein content via regulating the protein kinase B/target of rapamycin signalling pathway and (4) decreased malondialdehyde and protein carbonyl contents, increased catalase, glutathione-S-transferase, glutathione reductase and GSH activities, up-regulated mRNA levels of antioxidant enzymes related to NFE2-related factor 2 and γ-glutamylcysteine ligase catalytic subunit. These results suggest that Thr has a potential role to improve muscle growth and protein synthesis, which might be due to the regulation of GH-IGF system, muscle growth-related gene, antioxidative capacity and protein synthesis-related signalling pathways. Based on the quadratic regression analysis of specific growth rate, the Thr requirement of hybrid catfish (14·19-25·77 g) was estimated to be 13·77 g/kg of the diet (33·40 g/kg of dietary protein).

The thyroid hormone activating enzyme, type 2 deiodinase, induces myogenic differentiation by regulating mitochondrial metabolism and reducing oxidative stress

Thyroid hormone (TH) is a key metabolic regulator that acts by coordinating short- and long-term energy needs. Accordingly, significant metabolic changes are observed depending on thyroid status. Although it is established that hyperthyroidism augments basal energy consumption, thus resulting in an enhanced metabolic state, the net effects on cellular respiration and generation of reactive oxygen species (ROS) remain unclear. To elucidate the effects of augmented TH signal in muscle cells, we generated a doxycycline-inducible cell line in which the expression of the TH-activating enzyme, type 2 deiodinase (D2), is reversibly turned on by the "Tet-ON" system. Interestingly, increased intracellular TH caused a net shift from oxidative phosphorylation to glycolysis and a consequent increase in the extracellular acidification rate. As a result, mitochondrial ROS production, and both the basal and doxorubicin-induced production of cellular ROS were reduced. Importantly, the expression of a set of antioxidant genes was up-regulated, and, among them, the mitochondrial scavenger Sod2 was specifically induced at transcriptional level by D2-mediated TH activation. Finally, we observed that attenuation of oxidative stress and increased levels of SOD2 are key elements of the differentiating cascade triggered by TH and D2, thereby establishing that D2 is essential in coordinating metabolic reprogramming of myocytes during myogenic differentiation. In conclusion, our findings indicate that TH plays a key role in oxidative stress dynamics by regulating ROS generation. Our novel finding that TH and its intracellular metabolism act as mitochondrial detoxifying agents sheds new light on metabolic processes relevant to muscle physiology.

Hepatic differentiation of human pluripotent stem cells by developmental stage-related metabolomics products

Endogenous cell signals regulate tissue homeostasis and are significant for directing the fate of stem cells. During liver development, cytokines released from various cell types are critical for stem/progenitor cell differentiation and lineage expansions. To determine mechanisms in these stage-specific lineage interactions, we modeled potential effects of soluble signals derived from immortalized human fetal liver parenchymal cells on stem cells, including embryonic and induced pluripotent stem cells. For identifying lineage conversion and maturation, we utilized conventional assays of cell morphology, gene expression analysis and lineage markers. Molecular pathway analysis used functional genomics approaches. Metabolic properties were analyzed to determine the extent of hepatic differentiation. Cell transplantation studies were performed in mice with drug-induced acute liver failure to elicit benefits in hepatic support and tissue regeneration. These studies showed signals emanating from fetal liver cells induced hepatic differentiation in stem cells. Gene expression profiling and comparison of regulatory networks in immature and mature hepatocytes revealed stem cell-derived hepatocytes represented early fetal-like stage. Unexpectedly, differentiation-inducing soluble signals constituted metabolomics products and not proteins. In stem cells exposed to signals from fetal cells, mechanistic gene networks of upstream regulators decreased pluripotency, while simultaneously inducing mesenchymal and epithelial properties. The extent of metabolic and synthetic functions in stem cell-derived hepatocytes was sufficient for providing hepatic support along with promotion of tissue repair to rescue mice in acute liver failure. During this rescue, paracrine factors from transplanted cells contributed in stimulating liver regeneration. We concluded that hepatic differentiation of pluripotent stem cells with metabolomics products will be significant for developing therapies. The differentiation mechanisms involving metabolomics products could have an impact on advancing recruitment of stem/progenitor cells during tissue homeostasis.

YAP1 enhances NF-κB-dependent and independent effects on clock-mediated unfolded protein responses and autophagy in sarcoma

Terminal differentiation opposes proliferation in the vast majority of tissue types. As a result, loss of lineage differentiation is a hallmark of aggressive cancers, including soft tissue sarcomas (STS). Consistent with these observations, undifferentiated pleomorphic sarcoma (UPS), an STS subtype devoid of lineage markers, is among the most lethal sarcomas in adults. Though tissue-specific features are lost in these mesenchymal tumors they are most commonly diagnosed in skeletal muscle, and are thought to develop from transformed muscle progenitor cells. We have found that a combination of HDAC (Vorinostat) and BET bromodomain (JQ1) inhibition partially restores differentiation to skeletal muscle UPS cells and tissues, enforcing a myoblast-like identity. Importantly, differentiation is partially contingent upon downregulation of the Hippo pathway transcriptional effector Yes-associated protein 1 (YAP1) and nuclear factor (NF)-κB. Previously, we observed that Vorinostat/JQ1 inactivates YAP1 and restores oscillation of NF-κB in differentiating myoblasts. These effects correlate with reduced tumorigenesis, and enhanced differentiation. However, the mechanisms by which the Hippo/NF-κB axis impact differentiation remained unknown. Here, we report that YAP1 and NF-κB activity suppress circadian clock function, inhibiting differentiation and promoting proliferation. In most tissues, clock activation is antagonized by the unfolded protein response (UPR). However, skeletal muscle differentiation requires both Clock and UPR activity, suggesting the molecular link between them is unique in muscle. In skeletal muscle-derived UPS, we observed that YAP1 suppresses PERK and ATF6-mediated UPR target expression as well as clock genes. These pathways govern metabolic processes, including autophagy, and their disruption shifts metabolism toward cancer cell-associated glycolysis and hyper-proliferation. Treatment with Vorinostat/JQ1 inhibited glycolysis/MTOR signaling, activated the clock, and upregulated the UPR and autophagy via inhibition of YAP1/NF-κB. These findings support the use of epigenetic modulators to treat human UPS. In addition, we identify specific autophagy, UPR, and muscle differentiation-associated genes as potential biomarkers of treatment efficacy and differentiation.

Age-related changes in skeletal muscle: changes to life-style as a therapy

As we age, there is an age-related loss in skeletal muscle mass and strength, known as sarcopenia. Sarcopenia results in a decrease in mobility and independence, as well as an increase in the risk of other morbidities and mortality. Sarcopenia is therefore a major socio-economical problem. The mechanisms behind sarcopenia are unclear and it is likely that it is a multifactorial condition with changes in numerous important mechanisms all contributing to the structural and functional deterioration. Here, we review the major proposed changes which occur in skeletal muscle during ageing and highlight evidence for changes in physical activity and nutrition as therapeutic approaches to combat age-related skeletal muscle wasting.

Loss of tafazzin results in decreased myoblast differentiation in C2C12 cells: A myoblast model of Barth syndrome and cardiolipin deficiency

Barth syndrome (BTHS) is an X-linked genetic disorder resulting from mutations in the tafazzin gene (TAZ), which encodes the transacylase that remodels the mitochondrial phospholipid cardiolipin (CL). While most BTHS patients exhibit pronounced skeletal myopathy, the mechanisms linking defective CL remodeling and skeletal myopathy have not been determined. In this study, we constructed a CRISPR-generated stable tafazzin knockout (TAZ-KO) C2C12 myoblast cell line. TAZ-KO cells exhibit mitochondrial deficits consistent with other models of BTHS, including accumulation of monolyso-CL (MLCL), decreased mitochondrial respiration, and increased mitochondrial ROS production. Additionally, tafazzin deficiency was associated with impairment of myocyte differentiation. Future studies should determine whether alterations in myogenic determination contribute to the skeletal myopathy observed in BTHS patients. The BTHS myoblast model will enable studies to elucidate mechanisms by which defective CL remodeling interferes with normal myocyte differentiation and skeletal muscle ontogenesis.

Skeletal Muscle Pathophysiology: The Emerging Role of Spermine Oxidase and Spermidine

Skeletal muscle comprises approximately 40% of the total body mass. Preserving muscle health and function is essential for the entire body in order to counteract chronic diseases such as type II diabetes, cardiovascular diseases, and cancer. Prolonged physical inactivity, particularly among the elderly, causes muscle atrophy, a pathological state with adverse outcomes such as poor quality of life, physical disability, and high mortality. In murine skeletal muscle C2C12 cells, increased expression of the spermine oxidase (SMOX) enzyme has been found during cell differentiation. Notably, SMOX overexpression increases muscle fiber size, while SMOX reduction was enough to induce muscle atrophy in multiple murine models. Of note, the SMOX reaction product spermidine appears to be involved in skeletal muscle atrophy/hypertrophy. It is effective in reactivating autophagy, ameliorating the myopathic defects of collagen VI-null mice. Moreover, spermidine treatment, if combined with exercise, can affect D-gal-induced aging-related skeletal muscle atrophy. This review hypothesizes a role for SMOX during skeletal muscle differentiation and outlines its role and that of spermidine in muscle atrophy. The identification of new molecular pathways involved in the maintenance of skeletal muscle health could be beneficial in developing novel therapeutic lead compounds to treat muscle atrophy.

Role of miR-200c in Myogenic Differentiation Impairment via p66Shc: Implication in Skeletal Muscle Regeneration of Dystrophic mdx Mice

Duchenne muscular dystrophy (DMD) is a genetic disease associated with mutations of Dystrophin gene that regulate myofiber integrity and muscle degeneration, characterized by oxidative stress increase. We previously published that reactive oxygen species (ROS) induce miR-200c that is responsible for apoptosis and senescence. Moreover, we demonstrated that miR-200c increases ROS production and phosphorylates p66Shc in Ser-36. p66Shc plays an important role in muscle differentiation; we previously showed that p66Shc^−/− muscle satellite cells display lower oxidative stress levels and higher proliferation rate and differentiated faster than wild-type (wt) cells. Moreover, myogenic conversion, induced by MyoD overexpression, is more efficient in p66Shc^−/− fibroblasts compared to wt cells. Herein, we report that miR-200c overexpression in cultured myoblasts impairs skeletal muscle differentiation. Further, its overexpression in differentiated myotubes decreases differentiation indexes. Moreover, anti-miR-200c treatment ameliorates myogenic differentiation. In keeping, we found that miR-200c and p66Shc Ser-36 phosphorylation increase in mdx muscles. In conclusion, miR-200c inhibits muscle differentiation, whereas its inhibition ameliorates differentiation and its expression levels are increased in mdx mice and in differentiated human myoblasts of DMD. Therefore, miR-200c might be responsible for muscle wasting and myotube loss, most probably via a p66Shc-dependent mechanism in a pathological disease such as DMD.

Diabetes-Induced Dysfunction of Mitochondria and Stem Cells in Skeletal Muscle and the Nervous System

Diabetes mellitus is one of the most common metabolic diseases spread all over the world, which results in hyperglycemia caused by the breakdown of insulin secretion or insulin action or both. Diabetes has been reported to disrupt the functions and dynamics of mitochondria, which play a fundamental role in regulating metabolic pathways and are crucial to maintain appropriate energy balance. Similar to mitochondria, the functions and the abilities of stem cells are attenuated under diabetic condition in several tissues. In recent years, several studies have suggested that the regulation of mitochondria functions and dynamics is critical for the precise differentiation of stem cells. Importantly, physical exercise is very useful for preventing the diabetic alteration by improving the functions of both mitochondria and stem cells. In the present review, we provide an overview of the diabetic alterations of mitochondria and stem cells and the preventive effects of physical exercise on diabetes, focused on skeletal muscle and the nervous system. We propose physical exercise as a countermeasure for the dysfunction of mitochondria and stem cells in several target tissues under diabetes complication and to improve the physiological function of patients with diabetes, resulting in their quality of life being maintained.

The physiopathologic role of oxidative stress in skeletal muscle

Muscle senescence is a complex mechanism that is usually associated with a decrease in mass, strength and velocity of contraction. This state, known as sarcopenia, is a multifactorial process and it may be the consequence of several events, including accumulation of oxidative stress. The role of oxidative stress in the physiopathology of skeletal muscle is quite complex. Transiently increased levels of oxidative stress might reflect a potentially health promoting process, while an uncontrolled accumulation might have pathological implication. The physiopathological role of oxidative stress on skeletal muscle, its involvement in aging-induced sarcopenia, and potential countermeasures will be discussed.

Effects of Antioxidant Supplements on the Survival and Differentiation of Stem Cells

Although physiological levels of reactive oxygen species (ROS) are required to maintain the self-renewal capacity of stem cells, elevated ROS levels can induce chromosomal aberrations, mitochondrial DNA damage, and defective stem cell differentiation. Over the past decade, several studies have shown that antioxidants can not only mitigate oxidative stress and improve stem cell survival but also affect the potency and differentiation of these cells. Further beneficial effects of antioxidants include increasing genomic stability, improving the adhesion of stem cells to culture media, and enabling researchers to manipulate stem cell proliferation by using different doses of antioxidants. These findings can have several clinical implications, such as improving neurogenesis in patients with stroke and neurodegenerative diseases, as well as improving the regeneration of infarcted myocardial tissue and the banking of spermatogonial stem cells. This article reviews the cellular and molecular effects of antioxidant supplementation to cultured or transplanted stem cells and draws up recommendations for further research in this area.

Cytoprotective propensity of green tea polyphenols against citrinin-induced skeletal-myotube damage in C2C12 cells

The mycotoxin citrinin, is produced by several species of Penicillium, Aspergillus and Monascus, and is capable of inducing cytotoxicity, oxidative stress and apoptosis. The aim of the present study was to investigate the effect of citrinin in mouse skeletal muscle cells (C2C12) and to overcome the cellular adverse effects by supplementing green tea extract (GTE) rich in polyphenols. C2C12 myoblasts were differentiated to myotubes and were exposed to citrinin in a dose dependent manner (0-100 µM) for 24 h and IC₅₀ value was found to be 100 µM that resulted in decreased cell viability, increased LDH leakage and compromised membrane integrity. Mitochondrial membrane potential loss, increased accumulation of intracellular ROS and sub G1 phase of cell cycle was observed. To ameliorate the cytotoxic effects of CTN, C2C12 cells were pretreated with GTE (20, 40, 80 µg/ml) for 2 h followed by citrinin (100 µM) treatment for 24 h. GTE pretreatment combated citrinin-induced cytotoxicity and oxidative stress. GTE at 40 and 80 µg/ml significantly promoted cell survival and upregulated antioxidant enzyme activities (CAT, SOD, GPx) and endogenous antioxidant GSH, while the gene and protein expression levels were significantly restored through its effective antioxidant mechanism. Present study results suggested the antioxidant properties of GTE as a herbal source in ameliorating the citrinin-induced oxidative stress.

N-methyl-D-aspartate (NMDA) impairs myogenesis in C2C12 cells

INTRODUCTION

N-methyl-d-aspartate (NMDA) is expressed in sensory neurons and plays important roles in peripheral pain mechanisms. The aim of this study was to examine the effects and molecular mechanisms of NMDA on C2C12 myoblast proliferation and differentiation.

METHODS

Cytotoxicity and differentiation were examined by the MTT assay, reverse transcription-polymerase chain reaction, and immunofluorescence.

RESULTS

NMDA had no cytotoxicity (10-500 μM) and inhibited myoblastic differentiation of C2C12 cells, as assessed by F-actin immunofluorescence and levels of mRNAs encoding myogenic markers such as myogenin and myosin heavy-chain 2. It inhibited phosphorylation of mammalian target of rapamycin (mTOR) by inactivating mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38. It induced reactive oxygen species production. Furthermore, NMDA-suppressed expression of F-actin was reversed by adding the antioxidant N-acetylcysteine.

CONCLUSIONS

Collectively, these results indicate that NMDA impairs myogenesis or myogenic differentiation in C2C12 cells through the mTOR/MAPK signaling pathways and may lead to skeletal muscle degeneration. Muscle Nerve 56: 510-518, 2017.