Cdh2, a downstream target of Hes7, regulates somitogenesis by supporting FGF signalling

In the segmentation clock, Hes7 expression oscillates synchronously in the presomitic mesoderm (PSM), regulating periodic somite formation. Despite intensive studies, the whole regulatory gene networks of the segmentation clock remain to be analysed. To identify the direct target genes of Hes7, we performed chromatin immunoprecipitation with sequencing analysis using an anti-Hes7 antibody and knocked out the identified genes from mouse embryonic stem cells carrying a Hes7 reporter. These cells were induced to differentiate into PSM-like tissue, and live imaging of Hes7 oscillations was conducted. Among the tested genes, Cdh2 knockout resulted in downregulation of fibroblast growth factor (FGF) signalling and premature cessation of Hes7 oscillations. Conversely, Cdh2 overexpression led to upregulation of FGF signalling and prolonged Hes7 oscillations. Whereas Cdh2 mRNA showed dynamic expression through repression by Hes7 oscillations, Cdh2 protein exhibited a rather steady gradient with higher levels in the posterior PSM and lower levels in the anterior PSM. Thus, Hes7-controlled Cdh2 regulates FGF signalling, leading to the proper maintenance of Hes7 oscillations, suggesting that the interplay between Hes7 and Cdh2 governs the timing mechanism of PSM differentiation.