Number of vertebrae is fine-tuned by Notch signaling via control of period of the somite segmentation clock.. Mus musculus

The number of vertebrae is strictly defined for any given species1, and depends on the number of somites, which are periodically formed as perfectly matched cell masses during mid-embryogenesis, where the somite segmentation clock prescribes the timing2. The tempo of the clock is affected by surrounding condition, despite which the same number of somites is formed3, suggesting that the clock may be tunable to adapt to the environmental condition. Here, we demonstrate a tunability of the segmentation clock in the period depending on the level of Notch signaling that senses the surrounding information to adjust the number of somites and vertebrae precisely, in which we propose a mechanism of the clock with a feedback loop of Notch signaling by Notch-regulated ankyrin repeat protein (Nrarp). Disruption of Nrarp in mouse resulted in the loss of two vertebrae, due to 4-min extension of the period of the clock elicited by the up-regulation of Notch activity, whereas pharmacological diminishment of Notch activity shortens it by a few min. The Notch inhibitor rescues the phenotype of Nrarp knockout mice in the period. These results are comprehended by mathematical analyses, in which the period of the clock is fine-tuned by Notch activity that Nrarp adjusts. Overall, our results are the first to provide molecular evidence of fine-tuning of the segmentation clock that preserves the number of somites and vertebrae. Overall design: Nrarp mutant mouse which have LacZ gene instead of Nrarp coding region was generated by homologous recombinant. Fragnant female heterozygous mutant mice, mated with heterozygous mutant male, were dissected at embryonic day 10.5 (E10.5). 12 PSMs were collected in each genotype to extract total RNA.