Relationships between ultrastructural profiles and signal transduction of bone and skeletal muscle cells following development and growth

尽管生长因子和细胞因子在骨骼肌再生中发挥重要作用，但在再生肌细胞中激活这些因子的细胞内信号分子尚未阐明，一些证据表明白血病抑制因子（LIF）是体外成肌细胞增殖和存活以及加速骨骼肌细胞再生的重要细胞因子。增殖细胞核抗原（proliferating cell nuclear antigen，PCNA）和细胞周期蛋白依赖性激酶抑制剂p21（cyclin-dependent kinase inhibitor，p21）蛋白也是体外诱导肌细胞生长的关键分子。我们用免疫组织化学方法检测了这些信号分子在再生和功能过载的大鼠骨骼肌细胞中的时空激活模式。在肌纤维再生的早期，STAT 3蛋白首先在活化的卫星细胞核中被检测到，然后在增殖的同时表达PCNA和MyoD蛋白的成肌细胞中被激活，随着肌细胞再生的进行，在分化的成肌细胞和肌管中信号转导和转录激活因子3（STAT 3）蛋白不再被激活。此外，在未显示坏死迹象的存活肌纤维的完整肌膜内的肌核中也检测到STAT 3的激活。MyoD和myogenin蛋白在这些细胞中的持续表达是诱导骨骼肌细胞成功肥大的重要分子事件。

Although growth factors and cytokines play significant roles in skeletal muscle regeneration, intracellular signaling molecules that are activated these factors in regenerating muscle cells have been not elucidated.Several lines of evidence suggest that leukemia inhibitory factor(LIF)is an important cytokines for the proliferation and survival of myoblasts in vitro and acceleration of skeletal muscle cell regeneration.MyoD, myogenin, proliferating cell nuclear antigen(PCNA)and cyclin-dependent kinase inhibitor p21(p21)proteins are also key molecules in inducing the growth of muscle cells in vitro.To elucidate the role of these factors signaling in regenerative responses of skeletal muscle cells, we examined the spatial and temporal activation patterns of these signaling molecules in regenerating and in functionally overloaded rat skeletal muscle cells using immunohistochemical procedures.MyoD and myogenin were detected in myonuclei located inside the dystrophin-positive plasma membrane of myofibers, m-cadherin-positive satellite cell nuclei and nuclei located in the interstitial spaces between myofibers.At the early stage of regeneration, activated STAT3 proteins were first detected in the nuclei of activated satellite cells and then continued to be activated in proliferating myoblasts expressing both PCNA and MyoD proteins.When muscle cell regeneration processed, the signal transducer and activator transcription 3(STAT3)proteins was no longer activated in differentiated myoblasts and myotubes.In addition, activation of STAT3 was also detected in myonuclei within intact sarcolemmas of surviving myofibers that did not show signs of necrosis.Activation of STAT3 signaling is an important molecular event that induces the successful regeneration of skeletal muscle cells.And continual expression of MyoD and myogenin proteins in these cells is an essential molecular event which induces the successful hypertrophy of skeletal muscle cells.