Mechanism of somite segmentation by bHLH transcription factors

体是脊椎动物胚胎的异聚单位，是由分子钟调控的周期性分割产生的。Hes7是体裂所必需的bHLH基因，在体前中胚层（PSM）中循环表达，其2小时周期与体裂形成同步。Hes7对于其他基因如Lunatic fringe （lfg）的振荡表达也是必不可少的。Hes7的循环表达可能参与了somite segmentation的分子钟。因此，我们分析了Hes7振荡表达的机制。我们发现Hes7蛋白也以动态方式表达，这取决于蛋白酶体介导的降解。空间比较发现，Hes7和Lfng转录发生在Hes7蛋白阴性结构域。此外，Hes7和lfg的转录在缺乏Hes7蛋白时组成性上调，而在Hes7蛋白稳定时则下调。因此，Hes7蛋白的周期性抑制对Hes7和lfg的循环转录至关重要，这种负反馈代表了分割时钟的分子基础。

Somites, the metameric units of vertebrate embryos, are generated by periodical segmentation, which is regulated by a molecular clock. Hes7, a bHLH gene essential for somite segmentation, displays cyclic expression in the presomitic mesoderm (PSM) with 2 h cycles synchronized with somite formation. Hes7 is also essential for the oscillatory expression of the other gene such as Lunatic fringe (Lfng). It is likely that the cyclic expression of Hes7 is involved in the molecular clock in somite segmentation. Thus, we analyzed the mechanism of the oscillatory expression of Hes7.We show that Hes7 protein is also expressed in a dynamic manner, which depends on proteasome-mediated degradation. Spatial comparison revealed that Hes7 and Lfng transcription occurs in the Hes7 protein-negative domains. Furthermore, Hes7 and Lfng transcription is constitutively up-regulated in the absence of Hes7 protein and down-regulated by stabilization of Hes7 protein. Thus, periodic repression by Hes7 protein is critical for the cyclic transcription of Hes7 and Lfng, and this negative feedback represents a molecular basis for the segmentation clock.