TRANSCRIPTIONAL REGULATION OF SKELETAL MUSCLE-SPECIFIC GENES

骨骼肌的表型显著受 它接受运动神经元的神经支配。神经的模式和频率- 诱发的电活动改变受体的分布 肌管表面和肌纤维的收缩性能 调节基因转录。我们之前演示过MyoD 转录因子家族调控编码基因的表达 烟碱型乙酰胆碱受体(NAChR)亚单位和收缩 在肌肉分化过程中的蛋白质，以及它们可能解释 肌肉神经支配过程中受体基因的下调。vbl.使用 在原位杂交中，我们发现nAChR和生肌细胞水平 在整个神经支配的肌原纤维中，因子转录水平较低，并且 生肌因子mRNAs未与受体RNAs共定位 在突触核团。去神经后两天，所有这些文字记录 聚集在肌原纤维核中。此外，我们发现， 失神经损伤后分离的细胞核中肌生成素蛋白水平较高 肌肉多于神经支配的肌肉，这与 肌源性因子介导心肌梗死后受体基因上调 神经丧失。接下来，我们调查了不同刺激的效果 肌源性因子表达频率的变化。Northern印迹 从失神经肌肉中提取的RNA的分析 去神经和细胞外电极刺激，表明 MyoD因子家族受去极化频率下调 典型的快或慢抽动肌肉。与之相反， 编码肌钙蛋白I异构体的基因，这是参与 赋予肌肉快收缩和慢收缩特性的是 仅通过相应类型的刺激选择性上调 频率；去神经下调了它们的表达。这些结果 提示MyoD家族不能决定纤维类型的特异性 不同的因素控制着这些特性。我们已经开始 肌生成素和TnI慢基因调控序列的特征 以确定可抑制或激活的元素 对神经支配作出反应的转录。我们在转基因小鼠身上发现 3.7kb的肌生成素基因上游序列赋予肌肉特异性 转录到CAT报告基因和对 神经丧失。

Skeletal muscle phenotype is dramatically influenced by the type of motoneuron innervation it receives. The pattern and frequency of nerve- elicited electrical activity modifies the distribution of receptors on the myotube surface and the contractile properties of the myofiber by regulating gene transcription. We previously demonstrated that the MyoD family of transcription factors regulate the expression of genes coding for nicotinic acetylcholine receptor (nAChR) subunits and contractile proteins during muscle differentiation, and that they may account for the down regulation of receptor genes during muscle innervation. Using in situ hybridization, we found that the levels of nAChR and myogenic factor transcripts were lower throughout the innervated myofibril, and that the myogenic factor mRNAs were not co-localized with receptor RNAs at synaptic nuclei. Two days after denervation, all these transcripts accumulated in the myofibril nuclei. In addition, we found that the levels of myogenin protein are higher in nuclei isolated from denervated muscle than innervated muscle, consistent with the idea that the myogenic factors mediate the up-regulation of receptor genes after denervation. Next, we investigated the effects of different stimulation frequencies on the expression of the myogenic factors. Northern blot analysis of RNA isolated from muscle that was either denervated, or denervated and stimulated with extracellular electrodes, showed that the MyoD family of factors is down-regulated by depolarization frequencies typical of fast- or slow-twitch muscle. In contrast, expression of the genes coding for troponin I isoforms, which are proteins involved in conferring the fast and slow contractile properties to muscle, was selectively up-regulated only by the corresponding type of stimulus frequency; denervation down-regulated their expression. These results suggest that the MyoD family does not determine fiber-type specificity and that different factors regulate these properties. We have begun to characterize the regulatory sequences of the myogenin and TnI slow gene to identify the elements that either confer repression or activation of transcription in response to innervation. We found in transgenic mice that 3.7 kb of myogenin gene upstream sequences impart muscle-specific transcription to a CAT reporter gene and partial responses to denervation.