Valdez M R, Richardson J A, Klein W H, Olson E N
Department of Molecular Biology, University of Texas Southwestern Medical Center at Dallas, 6000 Harry Hines Boulevard, Dallas, Texas, 75235-9148, USA.
Dev Biol. 2000 Mar 15;219(2):287-98. doi: 10.1006/dbio.2000.9621.
The basic helix-loop-helix (bHLH) transcription factors-MyoD, Myf5, myogenin, and MRF4-can each activate the skeletal muscle-differentiation program in transfection assays. However, their functions during embryogenesis, as revealed by gene-knockout studies in mice, are distinct. MyoD and Myf5 have redundant functions in myoblast specification, whereas myogenin and either MyoD or MRF4 are required for differentiation. Paradoxically, myoblasts from myogenin mutant or MyoD/MRF4 double-mutant neonates differentiate normally in vitro, despite their inability to differentiate in vivo, suggesting that the functions of the myogenic bHLH factors are influenced by the cellular environment and that the specific myogenic defects observed in mutant mice do not necessarily reflect essential functions of these factors. Understanding the individual roles of these factors is further complicated by their ability to cross-regulate one another's expression. To investigate the functions of Myf5 in the absence of contributions from other myogenic bHLH factors, we generated triple-mutant mice lacking myogenin, MyoD, and MRF4. These mice appear to contain a normal number of myoblasts, but in contrast to myogenin or MyoD/MRF4 mutants, differentiated muscle fibers fail to form in vivo and myoblasts from neonates of this triple-mutant genotype are unable to differentiate in vitro. These results suggest that physiological levels of Myf5 are insufficient to activate the myogenic program in the absence of other myogenic factors and suggest that specialized functions have evolved for the myogenic bHLH factors to switch on the complete program of muscle gene expression.
碱性螺旋-环-螺旋（bHLH）转录因子——肌分化因子（MyoD）、肌原纤维蛋白（Myf5）、肌细胞生成素（myogenin）和肌肉调节因子4（MRF4）——在转染实验中均可激活骨骼肌分化程序。然而，小鼠基因敲除研究显示，它们在胚胎发育过程中的功能各不相同。MyoD和Myf5在成肌细胞特化过程中具有冗余功能，而肌细胞生成素以及MyoD或MRF4中的任意一个对于分化都是必需的。矛盾的是，来自肌细胞生成素突变体或MyoD/MRF4双突变新生小鼠的成肌细胞在体外能够正常分化，尽管它们在体内无法分化，这表明生肌bHLH因子的功能受细胞环境影响，且在突变小鼠中观察到的特定生肌缺陷不一定反映这些因子的基本功能。这些因子能够相互交叉调节彼此的表达，这使得了解它们各自的作用变得更加复杂。为了在没有其他生肌bHLH因子作用的情况下研究Myf5的功能，我们构建了缺乏肌细胞生成素、MyoD和MRF4的三突变小鼠。这些小鼠的成肌细胞数量似乎正常，但与肌细胞生成素或MyoD/MRF4突变体不同，体内无法形成分化的肌纤维，且这种三突变基因型新生小鼠的成肌细胞在体外也无法分化。这些结果表明，在没有其他生肌因子的情况下，Myf5的生理水平不足以激活生肌程序，这表明生肌bHLH因子已经进化出专门的功能来开启完整的肌肉基因表达程序。
