• BMB Reports
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• v.49 no.6
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• pp.303-304
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• 2016

Myoblast fusion is important for skeletal muscle formation. Even though the knowledge of myoblast fusion mechanism has accumulated over the years, the initial signal of fusion is yet to be elucidated. Our study reveals the novel function of a phosphatidylserine (PS) receptor, stabilin-2 (Stab2), in the modulation of myoblast fusion, through the recognition of PS exposed on myoblasts. During differentiation of myoblasts, Stab2 expression is higher than other PS receptors and is controlled by calcineurin/NFAT signaling on myoblasts. The forced expression of Stab2 results in an increase in myoblast fusion; genetic ablation of Stab2 in mice causes a reduction in muscle size, as a result of impaired myoblast fusion. After muscle injury, muscle regeneration is impaired in Stab2-deficient mice, resulting in small myofibers with fewer nuclei, which is due to reduction of fusion rather than defection of myoblast differentiation. The fusion-promoting role of Stab2 is dependent on its PS-binding motif, and the blocking of PS-Stab2 binding impairs cell-cell fusion on myoblasts. Given our previous finding that Stab2 recognizes PS exposed on apoptotic cells for sensing as an "eat-me" signal, we propose that PS-Stab2 binding is required for sensing of a "fuse-me" signal as the initial signal of myoblast fusion.

• BMB Reports
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• v.28 no.3
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• pp.191-196
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• 1995

The role of polyamines in skeletal myoblast differentiation was investigated using the polyamine metabolic inhibitor methylglyoxal bis(guanylhydrazone)(MGBG). Concentrations of intracellular free spermidine and spermine increased 2 to 2.5-fold at the onset of myoblast fusion. The systhesis of actin, and creatine kinase activity both dramatically increased during myotube formation. However, MGBG at a concentration of 0.5 mM not only abolished the increase of intracellular free polyamines, but also reduced cell fusion to almost half the level of untreated cells, without noticeable morphological alteration. The production of actin, and creatine kinase activity were almost completely abolished by MGBG. The inhibition of myoblast fusion by MGBG was partially recovered with 0.1 mM of spermidine or spermine added externally. Results indicate that polyamines are necessary for normal myoblast differentiation. Since the first indication of myoblast differentiation is alignment of muscle cells and membrane fusion of adjacent cells, and since polyamine depletion completely inhibited the synthesis of actin, which might be associted with membranes, polyamine might be involved in myoblast differentiation through membrane reorganization events.

• The Korean Journal of Physiology and Pharmacology
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• v.7 no.6
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• pp.375-379
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• 2003

$Ca^{2+}$ influx appears to be important for triggering myoblast fusion. It remains, however, unclear how $Ca^{2+}$ influx rises prior to myoblast fusion. Recently, several studies suggested that NMDA receptors may be involved in $Ca^{2+}$ mobilization of muscle, and that $Ca^{2+}$ influx is mediated by NMDA receptors in C2C12 myoblasts. Here, we report that other types of ionotropic glutamate receptors, non-NMDA receptors (AMPA and KA receptors), are also involved in $Ca^{2+}$ influx in myoblasts. To explore which subtypes of non-NMDA receptors are expressed in C2C12 myogenic cells, RT-PCR was performed, and the results revealed that KA receptor subunits were expressed in both myoblasts and myotubes. However, AMPA receptor was not detected in myoblasts but expressed in myotubes. Using a $Ca^{2+}$ imaging system, $Ca^{2+}$ influx mediated by these receptors was directly measured in a single myoblast cell. Intracellular $Ca^{2+}$ level was increased by KA, but not by AMPA. These results were consistent with RT-PCR data. In addition, KA-induced intracellular $Ca^{2+}$ increase was completely suppressed by treatment of nifedifine, a L-type $Ca^{2+}$ channel blocker. Furthermore, KA stimulated myoblast fusion in a dose-dependent manner. CNQX inhibited not only KA-induced myoblast fusion but also spontaneous myoblast fusion. Therefore, these results suggest that KA receptors are involved in intracellular $Ca^{2+}$ increase in myoblasts and then may play an important role in myoblast fusion.

• The Korean Journal of Zoology
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• v.27 no.2
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• pp.73-84
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• 1984

In order to investigate the effect of neurotransmitter on myoblast differentiation in vitro, the effects of dopamine and epinephrine on myoblast fusion and on the intracellular cAMP level in cultured myoblasts were examined. Dopamine $(3\\times10^{-5}M)$ and epinephrine $(3\\times10^{-5}M)$, when added at 34 hr after cell plating, markedly inhibited myoblast fusion, and dopamine was more potent than epinephrine. Both dopamine and epinephrine had no effect on intracellular cAMP level. At the same time, exogeneous dbcAMP, $PGE_1$, and aspirin were used to examine whether cAMP is involved in myoblast differentiation. dbcAMP $(1\\times10^{-4}M)$ inhibited myoblast fusion, whereas $PGE_1 (3\\times10^{-6}M)$ had no inhibitory effect, rather enhancing myoblast fusion. Aspirin, an inhibitor of PG synthetase, was shown to inhibit myoblast fusion. Possible mechanism by which dopamine or epinephrine at a specific concentration used inhibits myoblast fusion is discussed.

• The Korean Journal of Zoology
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• v.27 no.3
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• pp.151-164
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• 1984

In order to investigate the mechanism of myoblast fusion during muscle differentiation in culture, the effect of muscle-conditioned medium on the fusion was studied and possible release from cultured myoblast cells of proteins which may be responsible for the promotion of myoblast fusion was analyzed. The muscle-conditioned medium showed a marked fusion-promoting activity in a dose-dependent fashion. THis fusion-promoting activity of the muscle-conditioned medium appeared to be due to the accumulation of at least two proteins which were released from the myoblast into the culture medium. These released proteins were analyzed by electrophoresis and autoradiography and found to have molecular weights of 45,000 and 65,000.

• The Korean Journal of Zoology
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• v.35 no.1
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• pp.29-36
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• 1992

The present study describes the production of monoclonal antibodies against cultured chick myoblast to pursue critical proteins in muscle cell fusion. Among a panel of monoclonal antibodies, three, Mll-3H 13, Mll-3Hl8 and Mll-3H35 were inhibited movblast fusion. A single 101-kDa antigen reactive with monoclonal antibody Mll-3H35 was detected by radioimmu-noprecipitation or by immunoblotting. During the course of myogenesis, the level of the protein remarkably decreased as the cells there differentiated. These results suggest that the protein platys a direct role in the process of myoblast fusion mechanism.

• The Korean Journal of Zoology
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• v.36 no.3
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• pp.433-438
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• 1993

In the previous paper (Choi et al., 1992), we found that a large but transient elevation in intracellular cGMP levels occur concomitant with the myoblast fusion. To establish the physiological significance of the elevation of cGMP levels, the change in guanylate cyclase activity dudng myoblast fusion and the correlation hetween various chemicals that may affect guanylate cyclase adivity and myoblast fusion were examined. Sodium nitroprusside, a nitric oxide-forming compound, induced a precocious fusion and increased guanylate cyclase activity compared to the control. Furthermore, L-NG-monomethyl arginine, specific inhibitor of L-arginine: nitric oxide synthase, inhibited the cell fusion in a dose-dependent manner, without affecting biochemical differentiation. On the basis of our present findings, we propose that the onset of myoblast fusion is somehow correlated with the rise in cellular cGMP levels that is regulated by the activation or inhibItIon of soluble guanylate cyclase, via as yet undefined mechanism but possibly through L-arginine: nitric oxide pathway.

• The Korean Journal of Zoology
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• v.32 no.2
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• pp.163-175
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• 1989

Transferrin (Tf) has been known to exert profound effect on the myoblast differentiation in uirto. Therefore, the changes in the amount and affinity of the Tf receptor would accompany the myoblast differentiation. To investigate this possibility, we exarnined the afteration pattern in the level of the Tf receptor during the myoblast fusion. The level of Tf receptor was assayed by measuring the bound 125 I-Tf onto the surface of cultured myoblasts, and it was known that the level of Tf receptor reached the maximum at about 12 hr before the initi ation of the myoblast fusion and decreased as the differentiation proceeded, and that the affinity of Tf receptor to Tf was also decreased. In addition, various inhibitiors of the myoblast fusion also influenced the level of the Tf receptor. Accroding to these results, it is postulated that the level of Tf receptor is highly regulated during the myoblast differentiation.

• Journal of Life Science
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• v.12 no.1
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• pp.55-60
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• 2002

The differentiation of skeletal muscle precursor cells in culture is marked by the transcriptional activation of muscle-specific genes and the morphological differentiation of myoblast into multinucleate myotube. In this study, we examined the effect of TSA (Trichostatin A) on WF-kB DNA binding activity and muscle cell fusion in the process of myogenesis. Under TSA treatment, C2C12 myoblast could not fuse to myotube and its NF-kB DNA binding activity was also blocked. To investigate whether these phenomenons were affected by TSA in direct or not, differentiation media (DM) used to differentiate cells without TSA was concentrated and added to C2C12 myoblast with TSA simultaneously. C2C12 myoblast was fused to myotube and NF-kB DNA binding activity was recovered. These results suggest that TSA affects on the differentiation of myoblast, perhaps through several factors, by inhibiting myoblst fusion and blocking NF-kB DNA binding activity.

• The Korean Journal of Zoology
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• v.31 no.3
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• pp.207-217
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• 1988

A myotrophic protein that seemed to he eseentiai for the hision of chick embryonic myoblasts in culture was isolated from chick embryo extrad and was found to be identical or at least similar to the iron-transporting protein, transferrin. Embryo extract seemed to contain, in addition to this myotrophic protein, a heat stable protein that inhibits the fusion of myoblasts. Iron seemed to he necessary for myoblasts to fuse and it was supposed that the role of the myotrophic protein m myoblast fusion is to supply iron to the cell. The numher of the myotrophic protein receptors on myoblast surface membrane decreased immediately after the start of myoblast fusion, supposedly due to the decreased need of iron after the fusion once commenced. It was estimated that endocytosis of myotrophic protein took about 10 minutes and one recycling about 2 hours. The accumulation of iron in myoblasts continued linearly with cultre time and endocytosis of the myotrophic protein occured at a constant rate.