Molecular genetic analysis of relationship between traveling wave and constant wave of molecular clock in somitogenesis

Mesp1和Mesp2是同源转录因子，在小鼠体发生过程中共表达于前分裂前期中胚层。在我们的传统的Mesp2基因缺失的小鼠中，仅Mesp2的丢失就会导致体细胞发生的完全中断，包括节段边缘的形成、吻尾图案的形成和体细胞中胚层的上皮化。这导致我们解释，Mesp2是唯一负责体细胞发生的基因。然而，我们的新的Mesp2敲入等位基因表现出显著的Mesp1上调。从新的等位基因中移除PGK-neo盒会导致Mesp1的定位和几个基因的表达，并在尾部区域形成体节。此外，Mesp1的基因剂量减少一个拷贝会扰乱体节的形成，证实Mesp1参与了救援事件。此外，我们发现，激活的Notch1敲入显著上调了Mesp1的表达，即使在没有Notch信号介质PSEN1的情况下也是如此。这表明激活的Notch1的PSEN1非依赖性效应主要归因于Mesp1的诱导。然而，我们也证实了Mesp2增强了前PSM中Notch1受体的表达。因此，Notch信号的激活和随后的抑制可能是条纹图案形成和边界形成的关键事件。

Mesp1 and Mesp2 are homologous transcription factors that are co-expressed in the anterior presomitic mesoderm (PSM) during mouse somitogenesis. The loss of Mesp2 alone in our conventional Mesp2-null mice results in the complete disruption of somitogenesis, including segment border formation, rostro-caudal patterning and epithelialization of somitic mesoderm. This has led us to interpret that Mesp2 is solely responsible for somitogenesis. Our novel Mesp2 knock-in alleles, however, exhibit a remarkable upregulation of Mesp1. Removal of the pgk-neo cassette from the new allele leads to localization of Mesp1 and several gene expression, and somite formation in the tail region. Moreover, a reduction in the gene dosage of Mesp1 by one copy disrupts somite formation, confirming the involvement of Mesp1 in the rescue events. Furthermore, we find that activated Notch1 knock-in significantly upregulates Mesp1 expression, even in the absence of a Notch signal mediator, Psen1. This indicates that the Psen1-independent effects of activated Notch1 are mostly attributable to the induction of Mesp1. However, we have also confirmed that Mesp2 enhances the expression of the Notch1 receptor in the anterior PSM. The activation and subsequent suppression of Notch signaling might thus be a crucial event for both stripe pattern formation and boundary formation.