• Animal cells and systems
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• v.1 no.1
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• pp.63-69
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• 1997

We have previously reported that NF-kB is involved in the regulation of nitric oxide synthase gene expression during differentiation of chick embryonic myoblasts. However, how NF-kB is timely activated during myogenesis remains elusive. One of the most prominent events in myogenesis is myoblast membrane fusion, which is accompanied with massive cytoskeletal reorganization. Here we show that the activity of NF-kB markedly increases in L6 rat myogenic cells that have just initiated morphological changes by treating nocodazole, a microtubule-disrupting agent. Furthermore, the induction of NF-kB activation was closely correlated with the myoblast fusion. In addition, a variety of agents that disrupt microtubules stimulated the myoblast fusion as well as the induction of NF-kB activation. In contrast, taxol, a microtubule-stabilizing agent, suppressed the induction of NF-kB activation and inhibited spontaneous differentiation of L6 cells as well. In addition, we found that the NF-KB in the cells consists of p50/p65 heterodimers. These results support the idea that reorganization of microtubule at early stages of differentiation plays a role as a signal for NF-KB activation during myogenesis.

• The Korean Journal of Zoology
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• v.24 no.4
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• pp.189-200
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• 1981

Drastic alterations in myogenesis could be induced by ultraviolet irradiation of the myogenic cells derived from 12 day old chick embryo skeletal muscle. The effects of irradiation on various aspects, including cell division, transformation to myotubes, and morphology of myoblasts and myotubes, were examined. Irradiated cells were smaller in size, and only few cells transformed resulting in smaller size of myotubes with a narrow width. Both the inhibiting actions to cell division and to fusion were more striking when irradiated at earlier stages after plating. As well, cell division and fusion were inhibited more effectively with increasing UV dose and excessive amount caused cell death. A lowering cell density was thought to account for the decrease in myogenesis and possible reasons for the decrease in the capacity for fusion were discussed in view of the results presented in this report and of the findings from other laboratories.

• The Korean Journal of Zoology
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• v.33 no.2
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• pp.158-165
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• 1990

Proteolytic activity of calpain was found to increase as myoblast fusion proceeds. At 60 hr after cell seeding, lis activity reached to a maximal level and then slighdy decreased thereafter. Similarly, the protein level of calpain reached to a maximal level just proir to the initiation of fusion and remained elevated upon prolonged culture as analyzed by immunoblol using anti-calpain antiserum. These results suggest that the synthesis of calpain is regulated during myogenesis and its proteolytic activity may be related with the process of myoblasts fusion.

• The Korean Journal of Zoology
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• v.33 no.1
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• pp.108-118
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• 1990

Fibronectin is a glycoprotein found in the extracellular matrix as well as in the serum, and has been known to exert pronouned effed on the myoblast fusion. Our previous studies have suggested that the decrease of fibronectin levels during myogenesis is due to the decreased availability of the receptor for the 28 kDa fragrnent of fibronetin. In the fusion-blocked myoblasts by EGTA, the levels of fibronetin and binding of 28 kDa fragment decreased but far less than the control level. In contrast, the levels of fibronetin and binding of 28 kDa fragment decreased to the control level in the myoblast released from the fusion block. On this account, we suggest that the decrease of fibronetin levels during myoblast fusion is closely associated with the loss or alteration of the receptor for 28 kDa fragment. Mild trypsin treatment decreased the binding of the 28 kDa fragment to the myoblasts significandy. Similarly, the presence of gangliosides in the binding media decreased the binding of the 28 kDa fragment in a dose-dependent manner. Furthermore, gel overlay of 125 I-28 kDa fragment on the SDS-PAGE of the myoblast homogenates revealed that the 28 kDa fragment bound to a 43 kDa protein and to gangliosides as well. These results suggest that myoblast fusion is correlated with decrease of the receptor for the 28 kDa fragment and that the receptor might be a glycoprotein that contains glyco-conjugate found in gangliosides.

• The Korean Journal of Zoology
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• v.26 no.4
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• pp.235-251
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• 1983

Several approaches have been made to access the mechanism of fusion in chick myoblast in vitro. Lactoperoxidase-catalyzed iodination was applied to labell cell surface proteins during myogenesis. Quantitative as well as qualitative changes were observed in $^131$I surface components of prefusion and postfusion cells. Two proteins with a molecular weight of 165K and 93K daltons were observed to appear at the onset of fusion as compared to prefusion stage. At the same time, 245K dalton protein decreased whereas the low molecular weight proteins increased consistently. The decrease of high molecular weight proteins appears to be associated with the cell cycle of myoblast during differentiation. The increased appearance of low molecular weight proteins might be due to the proteolytic cleavage of the high molecular weight proteins. Examination of intracellulr cAMP levels during fusion has revealed that a large but transient increase in cAMP occurs before the onset of fusion. This result suggests a causal relationship between the increase of cAMP and the onset of fusion, and further, that differentiating myoblasts are synthronized to a high degree. During the course of myoblast differentiation, at least four lowe molecular weight proteins, which different from major surface proteins iodinated, were identifiable in the culture medium. These proteins could be ascribed to be released from the membrane by proteolytic cleavage of surface proteins in the course of myoblast fusion. The significance of cell surface alterations and the released proteins during the fusion, the involvement of cAMP in the onset of fusion and the possibility that fusion is promoted by external factor(s) are discussed.