Expression Of Muscle-specific Proteins Research Articles

Persistent mdx diaphragm alterations are accompanied by increased expression and activity of calcium and muscle-specific proteins.

The mdx mouse model of Duchenne Muscular Dystrophy (DMD) presents sarcolemma instability and develops a mild multi-stage dystrophinopathy characterized by intense myonecrosis with inflammatory infiltrate at 4-weeks; muscular regeneration at 12-weeks and persistent fibrosis onwards. Mdx diaphragm muscle has a more severe phenotype with structural and functional deterioration that closely resembles the diaphragm impairment responsible for DMD human patients' morbidity. Herein, we compared calcium deposits, activity of calcium-related proteases, and expression of muscle-specific proteins in mdx diaphragm at 4-weeks and 12-weeks. We found increased calcium deposits mainly at 12-weeks, concomitant with high activity of calpains and matrix metalloprotease-9, but decreased expression of Myh4 (Myhc IIb) and Atp2a1 (SERCA1), and high expression of the myogenic regulatory factors Myod1 and Myog. Our results suggest that increased calcium deposits and persistent activity of calcium dependent proteases throughout the disease are involved in the degeneration and regeneration processes in the mdx diaphragm.

FoxO1 inhibition attenuate diabetes‐related vascular remodeling by reducing chronic inflammatory states

Diabetic vascular complications result in poor outcomes, even after optimized interventions. Forkhead transcriptional factor(FoxO1) contributes to diabetic cardiomyopathy, yet the role of FoxO1 in diabetic vascular complications is poorly understood. We found that arterial wall hypertrophy occurred in the carotid artery of diabetic rats, which was associated with significant increases in wall thickness and cross‐sectional area as compared to non‐diabetic control group. The immunofluorescence results showed that α‐smooth muscle specific protein expression was downregulated in diabetic carotid media which was restored by treatment with the selective FoxO1 inhibitor AS1842856. The mRNA expressions of pro‐inflammatory biomarkers IL‐1 and TNF‐α in diabetic carotid arteries were increased as compared with control and were decreased following AS1842856 treatment. Also, the NLRP3 protein expression in diabetic carotid arteries was increased as compared with control group and reverted by AS1842856 treatment. It is concluded that our findings identify FoxO1 as a critical trigger for diabetes‐related adverse vascular remodeling and define the FoxO1 as a novel checkpoint for NLRP3 inflammasome‐dependent low‐grade chronic inflammatory states during diabetic adverse vascular remodeling progression, which may provide a promising therapeutic target to treat the diabetic cardiovascular disease.Support or Funding InformationThe work was supported by the NSFC grant (81770831) and in part by the General Research Fund (17118619M, 17123718M, Research Grants Council of Hong Kong).

De novo dNTP production is essential for normal postnatal murine heart development

The building blocks of DNA, dNTPs, can be produced de novo or can be salvaged from deoxyribonucleosides. However, to what extent the absence of de novo dNTP production can be compensated for by the salvage pathway is unknown. Here, we eliminated de novo dNTP synthesis in the mouse heart and skeletal muscle by inactivating ribonucleotide reductase (RNR), a key enzyme for the de novo production of dNTPs, at embryonic day 13. All other tissues had normal de novo dNTP synthesis and theoretically could supply heart and skeletal muscle with deoxyribonucleosides needed for dNTP production by salvage. We observed that the dNTP and NTP pools in WT postnatal hearts are unexpectedly asymmetric, with unusually high dGTP and GTP levels compared with those in whole mouse embryos or murine cell cultures. We found that RNR inactivation in heart led to strongly decreased dGTP and increased dCTP, dTTP, and dATP pools; aberrant DNA replication; defective expression of muscle-specific proteins; progressive heart abnormalities; disturbance of the cardiac conduction system; and lethality between the second and fourth weeks after birth. We conclude that dNTP salvage cannot substitute for de novo dNTP synthesis in the heart and that cardiomyocytes and myocytes initiate DNA replication despite an inadequate dNTP supply. We discuss the possible reasons for the observed asymmetry in dNTP and NTP pools in WT hearts.

Bisphenol A and estradiol impede myoblast differentiation through down-regulating Akt signaling pathway

Bisphenol A (BPA), one of the most widespread endocrine disrupting chemicals, is known as an artificial estrogen, which interacts with estrogen receptor (ER). In this study, we investigated the effects of BPA and estradiol on myoblast differentiation and the underlying signaling mechanism. Exposure to BPA (0.01–1 μM) in mouse myoblast C2C12 cells attenuated myogenic differentiation via the reduced expression of muscle-specific genes, such as myosin heavy chain (MHC), MyoD, and Myogenin, without the alteration of cell proliferation and viability. BPA-exposed C2C12 myoblasts also showed a reduction of Akt phosphorylation ((37–61) %, p < 0.001), a key event for myogenesis. Similarly to BPA, estradiol (0.01–1 μM) reduced the expression of muscle-specific proteins and the formation of multinucleated myotubes, and attenuated the muscle differentiation-specific phosphorylation of Akt ((42–59) %, p < 0.001). We conclude that BPA and estradiol suppress myogenic differentiation through the inhibition of Akt signaling.

5-Azacytidine-mediated hMSC behavior on electrospun scaffolds for skeletal muscle regeneration.

Incomplete regeneration after trauma or muscular dysfunction is a common problem in muscle replacement therapies. Recent approaches in tissue engineering allow for the replication of skeletal muscle structure and function in vitro and in vivo by molecular therapies and implantable scaffolds which properly address muscle cells toward myotube differentiation and maturation. Here, we investigate the in vitro response of human mesenchymal stem cells (hMSC) on electrospun fibers made of polycaprolactone (PCL) in the presence of 5-azacytidine (5-AZA) to evaluate how fibrous network may influence the therapeutic effect of drug during in vitro myogenesis. Biological studies demonstrate the ability of hMSCs to differentiate in mature myofibers in supplemented (myogenic) and, preferentially, in 5-AZA-enriched culture. PCL electrospun fibers amplify the 5-AZA capability to induce a low proliferation rate in hMSC, thus promoting hMSC differentiation (MTT assay). Qualitative (Azan Mallory stain, immunofluorescence assay, SEM analyses) and quantitative (ELISA test) assays confirm the synergistic contribution of PCL electrospun fibers and 5-AZA on in vitro myotubes formation and maturation. This result is also confirmed by the expression of muscle-specific proteins related to the myogenic mechanisms in the presence of other muscle inductive signals (i.e., oxytocin, Tweak). Hence, we suggest the use of PCL electrospun fibers as interesting preclinical model to explore the effect of drugs and chemotherapeutics administration after damaged muscle resection. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2551-2561, 2017.

Increased IGF-IEc expression and mechano-growth factor production in intestinal muscle of fibrostenotic Crohn's disease and smooth muscle hypertrophy.

The igf1 gene is alternatively spliced as IGF-IEa and IGF-IEc variants in humans. In fibrostenotic Crohn's disease, the fibrogenic cytokine TGF-β1 induces IGF-IEa expression and IGF-I production in intestinal smooth muscle and results in muscle hyperplasia and collagen I production that contribute to stricture formation. Mechano-growth factor (MGF) derived from IGF-IEc induces skeletal and cardiac muscle hypertrophy following stress. We hypothesized that increased IGF-IEc expression and MGF production mediated smooth muscle hypertrophy also characteristic of fibrostenotic Crohn's disease. IGF-IEc transcripts and MGF protein were increased in muscle cells isolated from fibrostenotic intestine under regulation by endogenous TGF-β1. Erk5 and MEF2C were phosphorylated in vivo in fibrostenotic muscle; both were phosphorylated and colocalized to nucleus in response to synthetic MGF in vitro. Smooth muscle-specific protein expression of α-smooth muscle actin, γ-smooth muscle actin, and smoothelin was increased in affected intestine. Erk5 inhibition or MEF2C siRNA blocked smooth muscle-specific gene expression and hypertrophy induced by synthetic MGF. Conditioned media of cultured fibrostenotic muscle induced muscle hypertrophy that was inhibited by immunoneutralization of endogenous MGF or pro-IGF-IEc. The results indicate that TGF-β1-dependent IGF-IEc expression and MGF production in patients with fibrostenotic Crohn's disease regulates smooth muscle cell hypertrophy a critical factor that contributes to intestinal stricture formation.

Umbilical cord mesenchymal stem cell-conditioned media prevent muscle atrophy by suppressing muscle atrophy-related proteins and ROS generation.

The therapeutic potential of mesenchymal stem cell-conditioned medium (MSC-CM) has been reported with various types of disease models. Here, we examine the therapeutic effect of umbilical cord MSC-CM (UCMSC-CM) on muscle-related disease, using a dexamethasone (Dex)-induced muscle atrophy in vitro model. The expressions of muscle atrophy-related proteins (MuRF-1 and MAFbx) and muscle-specific proteins (desmin and myogenin) were evaluated by Western blot analysis. The level of production of reactive oxygen species (ROS) was determined using a 2',7'-dichlorofluorescein diacetate (DCFDA) dye assay. The expression of antioxidant enzymes (copper/zinc-superoxide dismutase (Cu/Zn-SOD), manganese superoxide dismutase (MnSOD), glutathione peroxidase-1 (GPx-1), and catalase (CAT)) was verified by reverse transcription polymerase chain reaction (RT-PCR). When L6 cells were exposed to Dex, the expression of muscle atrophy-related proteins was increased by 50-70%, and the expression of muscle-specific proteins was in turn decreased by 23-40%. Conversely, when the L6 cells were co-treated with UCMSC-CM and Dex, the expression of muscle atrophy-related proteins was reduced in a UCMSC-CM dose-dependent manner and the expression of muscle-specific proteins was restored to near-normal levels. Moreover, ROS generation was effectively suppressed and the expression of antioxidant enzymes was recovered to a normal degree. These data imply that UCMSC-CM clearly has the potential to prevent muscle atrophy. Thus, our present study offers fundamental data on the potential treatment of muscle-related disease using UCMSC-CM.

Inhibition of glycogen synthase kinase-3β attenuates glucocorticoid-induced suppression of myogenic differentiation in vitro.

Glucocorticoids are the only therapy that has been demonstrated to alter the progress of Duchenne muscular dystrophy (DMD), the most common muscular dystrophy in children. However, glucocorticoids disturb skeletal muscle metabolism and hamper myogenesis and muscle regeneration. The mechanisms involved in the glucocorticoid-mediated suppression of myogenic differentiation are not fully understood. Glycogen synthase kinase-3β (GSK-3β) is considered to play a central role as a negative regulator in myogenic differentiation. Here, we showed that glucocorticoid treatment during the first 48 h in differentiation medium decreased the level of phosphorylated Ser9-GSK-3β, an inactive form of GSK-3β, suggesting that glucocorticoids affect GSK-3β activity. We then investigated whether GSK-3β inhibition could regulate glucocorticoid-mediated suppression of myogenic differentiation in vitro. Two methods were employed to inhibit GSK-3β: pharmacological inhibition with LiCl and GSK-3β gene knockdown. We found that both methods resulted in enhanced myotube formation and increased levels of muscle regulatory factors and muscle-specific protein expression. Importantly, GSK-3β inhibition attenuated glucocorticoid-induced suppression of myogenic differentiation. Collectively, these data suggest the involvement of GSK-3β in the glucocorticoid-mediated impairment of myogenic differentiation. Therefore, the inhibition of GSK-3β may be a strategy for preventing glucocorticoid-induced muscle degeneration.

Small Leucine Zipper Protein (sLZIP) Negatively Regulates Skeletal Muscle Differentiation via Interaction with α-Actinin-4

The small leucine zipper protein (sLZIP) plays a role in transcriptional regulation in various types of cells. However, the role of sLZIP in myogenesis is unknown. We identified α-actinin-4 (ACTN4) as a sLZIP-binding protein. ACTN4 functions as a transcriptional regulator of myocyte enhancer factor (MEF)2, which plays a critical role in expression of muscle-specific genes during skeletal muscle differentiation. We found that ACTN4 translocates to the nucleus, induces myogenic gene expression, and promotes myotube formation during myogenesis. The myogenic process is controlled by an association between myogenic factors and MEF2 transcription factors. ACTN4 increased expression of muscle-specific proteins via interaction with MEF2. However, sLZIP decreased myogenic gene expression and myotube formation during myogenesis via disruption of the association between ACTN4 and MEF2. ACTN4 increased the promoter activities of myogenic genes, whereas sLZIP abrogated the effect of ACTN4 on transcriptional activation of myogenic genes in myoblasts. The C terminus of sLZIP is required for interaction with the C terminus of ACTN4, based on deletion mutant analysis, and sLZIP plays a role in regulation of MEF2 transactivation via interaction with ACTN4. Our results indicate that sLZIP negatively regulates skeletal muscle differentiation via interaction with ACTN4 and that sLZIP can be used as a therapeutic target molecule for treatment of muscle hypertrophy and associated diseases.

Expression profiles of muscle disease-associated genes and their isoforms during differentiation of cultured human skeletal muscle cells

BackgroundThe formation of contractile myofibrils requires the stepwise onset of expression of muscle specific proteins. It is likely that elucidation of the expression patterns of muscle-specific sarcomeric proteins is important to understand muscle disorders originating from defects in contractile sarcomeric proteins.MethodsWe investigated the expression profile of a panel of sarcomeric components with a focus on proteins associated with a group of congenital disorders. The analyses were performed in cultured human skeletal muscle cells during myoblast proliferation and myotube development.ResultsOur culture technique resulted in the development of striated myotubes and the expression of adult isoforms of the sarcomeric proteins, such as fast TnI, fast TnT, adult fast and slow MyHC isoforms and predominantly skeletal muscle rather than cardiac actin. Many proteins involved in muscle diseases, such as beta tropomyosin, slow TnI, slow MyBPC and cardiac TnI were readily detected in the initial stages of muscle cell differentiation, suggesting the possibility of an early role for these proteins as constituent of the developing contractile apparatus during myofibrillogenesis. This suggests that in disease conditions the mechanisms of pathogenesis for each of the mutated sarcomeric proteins might be reflected by altered expression patterns, and disturbed assembly of cytoskeletal, myofibrillar structures and muscle development.ConclusionsIn conclusion, we here confirm that cell cultures of human skeletal muscle are an appropriate tool to study developmental stages of myofibrillogenesis. The expression of several disease-associated proteins indicates that they might be a useful model system for studying the pathogenesis of muscle diseases caused by defects in specific sarcomeric constituents.

Synergistic stimulation of myogenesis by glucocorticoid and IGF-I signaling

Muscle wasting is associated with poor prognosis in chronic obstructive pulmonary disease (COPD). Exercise stimulates muscle recovery, but its efficacy is variable, depending on the clinical condition and medical treatment. Systemic glucocorticoids, commonly administered in high doses during acute disease exacerbations or as maintenance treatment in end-stage disease, are known to contribute to muscle wasting. As muscle mass recovery involves insulin-like growth factor (IGF)-I signaling, which can be stimulated by anabolic steroids, the impact of glucocorticoids and the effect of simultaneous IGF-I stimulation by anabolic steroids on muscle recovery and growth were investigated. The effects of, and interactions between, glucocorticoid and IGF-I signaling on skeletal muscle growth were assessed in differentiating C2C12 myocytes. As proof of principle, we performed a post hoc analysis stratifying patients by glucocorticoid use of a clinical trial investigating the efficacy of anabolic steroid supplementation on muscle recovery in muscle-wasted patients with COPD. Glucocorticoids strongly impaired protein synthesis signaling, myotube formation, and muscle-specific protein expression. In contrast, in the presence of glucocorticoids, IGF-I synergistically stimulated myotube fusion and myofibrillar protein expression, which corresponded with restored protein synthesis signaling by IGF-I and increased transcriptional activation of muscle-specific genes by glucocorticoids. In COPD patients on maintenance glucocorticoid treatment, the clinical trial also revealed an enhanced effect of anabolic steroids on muscle mass and respiratory muscle strength. In conclusion, synergistic effects of anabolic steroids and glucocorticoids on muscle recovery may be caused by relief of the glucocorticoid-imposed blockade on protein synthesis signaling, allowing effective translation of glucocorticoid-induced accumulation of muscle-specific gene transcripts.

Evaluation of serum‐free differentiation conditions for C2C12 myoblast cells assessed as to active tension generation capability

We have compared several serum-free media for the differentiation of C2C12 myoblasts and assessed the extent of differentiation in several ways including as to active tension generation capability. C2C12 cells were allowed to differentiate in Dulbecco's modified Eagle's medium (DMEM) containing Ham's F-12 (F-12), AIM-V (AIM), 0.2% Ultroser-G in DMEM (Ult-G), and 0.1% Sericin in DMEM (Sericin), compared with in DMEM supplemented with 2% horse serum (HS) or 2% calf serum (CS). C2C12 differentiation was assessed as the extent of myotube formation, glucose metabolism, protein expression, sarcomere formation, and active tension generation. All serum-free media examined were capable of inducing myotube formation and the expression of muscle-specific proteins. All serum-free media except for F-12 gave the sarcomere structure. Active tension generation was observed for cells that differentiated in AIM and Ult-G, but the active tension generated by C2C12 cells that differentiated in Ult-G was only ∼25% in the case of myotubes that formed in HS. The addition of Ult-G to the AIM resulted in improvement of the active tension generation capability, the active tension generated being ∼3.4× compared to that in HS. The approach for assessing muscle cell differentiation presented in this study will be suitable for other studies that involve the differentiation of muscle cells.

Arsenic Inhibits Myogenic Differentiation and Muscle Regeneration

BackgroundThe incidence of low birth weights is increased in offspring of women who are exposed to high concentrations of arsenic in drinking water compared with other women. We hypothesized that effects of arsenic on birth weight may be related to effects on myogenic differentiation.ObjectiveWe investigated the effects of arsenic trioxide (As2O3) on the myogenic differentiation of myoblasts in vitro and muscle regeneration in vivo.MethodsC2C12 myoblasts and primary mouse and human myoblasts were cultured in differentiation media with or without As2O3 (0.1–0.5 μM) for 4 days. Myogenic differentiation was assessed by myogenin and myosin heavy chain expression and multinucleated myotube formation in vitro; skeletal muscle regeneration was tested using an in vivo mouse model with experimental glycerol myopathy.ResultsA submicromolar concentration of As2O3 dose-dependently inhibited myogenic differentiation without apparent effects on cell viability. As2O3 significantly and dose-dependently decreased phosphorylation of Akt and p70s6k proteins during myogenic differentiation. As2O3-induced inhibition in myotube formation and muscle-specific protein expression was reversed by transfection with the constitutively active form of Akt. Sections of soleus muscles stained with hematoxylin and eosin showed typical changes of injury and regeneration after local glycerol injection in mice. Regeneration of glycerol-injured soleus muscles, myogenin expression, and Akt phosphorylation were suppressed in muscles isolated from As2O3-treated mice compared with untreated mice.ConclusionOur results suggest that As2O3 inhibits myogenic differentiation by inhibiting Akt-regulated signaling.

Urothelium-derived Sonic hedgehog promotes mesenchymal proliferation and induces bladder smooth muscle differentiation

Induction of smooth muscle differentiation from bladder mesenchyme depends on signals that originate from the urothelium. We hypothesize Sonic hedgehog (Shh) is the urothelial signal that promotes bladder mesenchymal proliferation and induces bladder smooth muscle differentiation. Pregnant FVB mice were euthanized on embryonic day (E) 12.5 and fetal bladders were harvested. Two experimental protocols were utilized: (1) Bladder mesenchyme (BLM) was isolated by incubating intact bladders (IB) in 0.02 M EDTA and then removing the urothelium by microdissection. IB and BLM were cultured in Shh-deficient media or BLM was cultured in Shh-supplemented (480 nM) media for 72 h. (2) IB were cultured for 72 h in media containing different concentrations of Shh (0, 48, and 480 nM). Specimens were sized by serial sectioning. Cell counts were performed after trypsin digestion. Immunohistochemistry was performed to detect smooth muscle-specific protein expression. α-Actin expression was quantified using Western blot. All specimens were viable at 72 h. BLM cultured without Shh survived but did not grow or undergo smooth muscle differentiation. IB cultured without Shh and BLM cultured with Shh grew and expressed smooth muscle proteins at 72 h. IB cultured with Shh were larger and contained more cells than IB cultured without Shh (all p<0.05). Increasing Shh concentration from 48 to 480 nM did not change bladder size, cell counts, or the level of α-actin expression. Prior to culture, IB did not express α-actin. After culture of IB in Shh-deficient media, α-actin was detected throughout the mesenchyme except in the submucosal layer. The IB submucosa was thinner after culture with 48 nM Shh and smooth muscle completely obliterated the submucosa after culture with 480 nM Shh. In fetal mouse bladders, urothelium-derived Shh is necessary for mesenchymal proliferation and smooth muscle differentiation. Shh concentration affects mesenchymal proliferation and patterning of bladder smooth muscle.

Differentiation-dependent PTPIP51 Expression in Human Skeletal Muscle Cell Culture

Protein tyrosine phosphatase-interacting protein 51 (PTPIP51) expression was analyzed in proliferating and differentiating human myogenic cells cultured in vitro. Satellite cell cultures derived from four different individuals were used in this study. To analyze the expression of PTPIP51, myoblasts were cultured under conditions promoting either proliferation or differentiation. In addition, further differentiation of already-differentiated myobtubes was inhibited by resubmitting the cells to conditions promoting proliferation. PTPIP51 protein and mRNA were investigated in samples taken at defined time intervals by immunostaining, immunoblotting, in situ hybridization, and PCR. Image analyses of fluorescence immunostainings were used to quantify PTPIP51 in cultured myoblasts and myotubes. Myoblasts grown in the presence of epidermal and fibroblast growth factors (EGF and FGF), both promoting proliferation, expressed PTPIP51 on a basic level. Differentiation to multinuclear myotubes displayed a linear increase in PTPIP51 expression. The rise in PTPIP51 protein was paralleled by an augmented expression of muscle-specific proteins, namely, sarcoplasmic reticulum Ca(2+) ATPase and myosin heavy-chain protein, both linked to a progressive state of myotubal differentiation. This differentiation-induced increase in PTPIP51 was partly reversible by resubmission of differentiated myotubes to conditions boosting proliferation. The results clearly point toward a strong association between PTPIP51 expression and differentiation in human muscle cells.

Stau1 negatively regulates myogenic differentiation in C2C12 cells

Sequential expression of myogenic regulatory factors (MRFs) such as MyoD and myogenin drives myogenic differentiation. Besides transcriptional activation of MRFs, this process is also coordinated by post-transcriptional regulation; MyoD and myogenin mRNAs are stabilized by RNA-binding protein HuR. Stau1 is known to regulate mRNA stability in a complex with Upf1, which is termed Stau1-mediated mRNA decay (SMD). We describe here that Stau1 is involved in the regulation of myogenesis. We found that knockdown of Stau1 promotes myogenesis including the expression of a muscle-specific marker protein, myoglobin, in C2C12 myoblasts. MyoD induces myogenin expression in response to induction of myogenesis, which is a key step to start myogenesis. The level of MyoD protein was not affected when Stau1 was depleted by siRNA, whereas the levels of myogenin mRNA and protein were increased in Stau1-knockdown cells. Interestingly, myogenin promoter activity was also increased in Stau1-knockdown cells in the absence of induction of myogenesis. Furthermore, Stau1-knockdown cells spontaneously progressed myogenesis including the expression of muscle-specific protein. Although Stau1 is involved in mRNA decay together with Upf1, Upf1-knockdown did not affect progression of myogenesis. Our results suggest that Stau1 negatively regulates myogenesis in C2C12 myoblasts through a mechanism that is different from SMD.

Regulation of Myoblast Motility and Fusion by the CXCR4-associated Sialomucin, CD164

Myoblast fusion is fundamental to the development and regeneration of skeletal muscle. To fuse, myoblasts undergo cell-cell recognition and adhesion and merger of membranes between apposing cells. Cell migration must occur in advance of these events to bring myoblasts into proximity, but the factors that regulate myoblast motility are not fully understood. CD164 is a cell surface sialomucin that is targeted to endosomes and lysosomes via its intracellular region. In hematopoietic progenitor cells, CD164 forms complexes with the motility-stimulating chemokine receptor, CXCR4, in response to the CXCR4 ligand, CXCL12/SDF-1 (Forde, S., Tye, B. J., Newey, S. E., Roubelakis, M., Smythe, J., McGuckin, C. P., Pettengell, R., and Watt, S. M. (2007) Blood 109, 1825-1833). We have previously shown that CD164 stimulates myotube formation in vitro. We report here that CD164 is associated with CXCR4 in C2C12 myoblasts. Cells in which CD164 levels are increased or decreased via overexpression or RNA interference-mediated knockdown, respectively, show enhanced or reduced myotube formation and cell migration, the latter both basally and in response to CXCL12/SDF-1. Furthermore, expression of CD164 cytoplasmic tail mutants that alter the endosome/lysosome targeting sequence and, consequently, the subcellular localization in myoblasts, reveals a similar correlation between cell motility and myotube formation. Finally, Cd164 mRNA is expressed in the dorsal somite (the early myogenic compartment of the mouse embryo) and in premuscle masses. Taken together, these results suggest that CD164 is a regulator of myoblast motility and that this property contributes to its ability to promote myoblast fusion into myotubes.

Cellular and molecular events controlling skeletal muscle mass in response to altered use

Gain or loss of skeletal muscle mass occurs in situations of altered use such as strength training, aging, denervation, or immobilization. This review examines our current understanding of the cellular and molecular events involved in the control of muscle mass under conditions of muscle use and disuse, with particular attention to the effects of resistance exercise/training. The DNA content, which is a critical determinant of protein synthesis by providing the amount of DNA necessary to sustain gene transcription, can be either increased (activation of satellite cells) or decreased (apoptosis) depending on muscle activity and ongoing physiological processes. In addition, several transcription factors are sensitive to functional demand and may control muscle-specific protein expression to promote or repress myofiber enlargement. The control of skeletal muscle mass is also markedly mediated by the regulation of transduction pathways that promote the synthesis and/or the degradation of proteins. Insulin-like growth factor-I plays a key role in this balance by activating the Akt/tuberous sclerosis complex 2/mammalian target of rapamycin pathway. Stimulation of this pathway leads to the concomitant activation of initiation and elongation factors resulting in the elevation of protein translation and the downregulation of ubiquitin proteasome components through Forkhead-box O transcription factors.

Promoting effect of 5-azacytidine on the myogenic differentiation of bone marrow stromal cells

Bone marrow stromal cells (BMSC) can differentiate into various cell types including myocytes, which may be valuable in cellular therapy of myocardial infarction. In an attempt to increase the myogenic commitment of BMSC, we investigated the extent of conversion induced by the demethylation agent 5-azacytidine. BMSC isolated from the adult rat tibia were exposed in culture to 5 μM 5-azacytidine for 24 h, 1 day after seeding. The treatment was repeated at weekly intervals and the expression of muscle-specific proteins and genes was assessed. The results revealed that cultured cells lost the native expression of osteocalcin and alkaline phosphatase as a function of time and began to express connexin 43. Exposure to 5-azacytidine of BMSC induced, at 14 days, a myocyte-resembling phenotype that included the expression of muscle-specific proteins (sarcomeric α-actin, troponin T, desmin, α-actinin, and GATA-4) and genes (GATA-4, myoD, desmin, and α-actinin), numerous mitochondria and myofilaments; however, the latter did not form sarcomeres. Although some of these myogenic markers also appeared in untreated cells, exposure to 5-azacytidine induced an enhanced response of calcium channels, as well as a threefold increase in desmin and myoD gene expression and a twofold increase in α-actinin gene and protein expression above the control values. In conclusion, the results demonstrate a promoting effect of 5-azacytidine on the expression of muscle-specific proteins and genes in BMSC in culture. Notably, the myogenic differentiation takes place over a short period of time. Priming of mesenchymal cells to cardiomyogenic differentiation may have significant applications in cellular approaches to ameliorate muscle loss after myocardial ischemia.

Effects of PAX3-FKHR on malignant phenotypes in alveolar rhabdomyosarcoma

The malignancy of alveolar rhabdomyosarcoma (ARMS) has been linked to expression of the PAX3-FKHR chimeric gene. To understand the effect of this gene, we used RNAi to knock down its expression (without affecting the expressions of either PAX3 or FKHR) in human ARMS cell lines. Down-regulating PAX3-FKHR caused (a) tumor cells to accumulate in the G1 phase, inhibiting the rate of cellular proliferation, (b) a reduction in the levels of the MET, reducing cell motility stimulated by HGF, and (c) induction of the myogenic differentiation gene, myogenin, and muscle differentiation (morphologic change and the expression of muscle specific proteins, desmin, and myosin heavy chain). These results suggest that PAX3-FKHR in ARMS cells promotes malignant phenotypes such as proliferation, motility, and to suppress differentiation.