Satoh N, Araki I, Satou Y
Department of Zoology, Graduate School of Science, Kyoto University, Japan.
Proc Natl Acad Sci U S A. 1996 Sep 3;93(18):9315-21. doi: 10.1073/pnas.93.18.9315.
The B-line presumptive muscle cells of ascidian embryos have extensive potential for self-differentiation dependent on determinants prelocalized in the myoplasm of fertilized eggs. Ascidian larval muscle cells therefore provide an experimental system with which to explore an intrinsic genetic program for autonomous specification of embryonic cells. Experiments with egg fragments suggested that maternal mRNAs are one of the components of muscle determinants. Expression of larval muscle actin genes begins as early as the 32-cell stage, prior to the developmental fate restriction of the cells. The timing of initiation of the actin gene expression proceeds the expression of an ascidian homologue of vertebrate MyoD by a few hours. Mutations in the proximal E-box of the 5' flanking region of the actin genes did not alter the promoter activity for muscle-specific expression of reporter gene. These results, together with results of deletion constructs of fusion genes, suggest that muscle determinants regulate directly, or indirectly via regulatory factors other than MyoD, the transcription of muscle-specific structural genes leading to the terminal differentiation.
海鞘胚胎的B线推定肌细胞具有广泛的自我分化潜能，这种潜能依赖于受精卵肌质中预先定位的决定因素。因此，海鞘幼虫肌细胞提供了一个实验系统，用以探索胚胎细胞自主特化的内在遗传程序。对卵片段的实验表明，母体mRNA是肌肉决定因素的组成部分之一。幼虫肌动蛋白基因的表达早在32细胞阶段就开始了，早于细胞发育命运的限制。肌动蛋白基因表达开始的时间比脊椎动物MyoD的海鞘同源物的表达提前几个小时。肌动蛋白基因5'侧翼区域近端E盒中的突变并未改变报告基因肌肉特异性表达的启动子活性。这些结果，连同融合基因缺失构建体的结果，表明肌肉决定因素直接或通过MyoD以外的调节因子间接调节导致终末分化的肌肉特异性结构基因的转录。
