Rac1 inhibition slows down the segmentation clock and increases the somite size

The presomitic mesoderm (PSM) is a transient embryonic tissue patterned along the anterior-posterior (A/P) axis of vertebrates that periodically generates somites at its anterior end. This process, termed somitogenesis, is driven by waves of gene expression that travel from the posterior to the anterior end of the PSM under the contol of the segmentation clock, a molecular oscillator. Using a primary culture system, we performed a small-scale chemical screen targeting cellular mechanical properties and we identified Rac1, a small GTPase, as a key regulator. Inhibition of Rac1 markedly altered the periodicity of gene expression waves and disrupted somite spatial organization. Further analyses showed that Rac1 inhibition modified gene expression patterns, notably affecting markers of the Wnt and Notch pathways which are central regulators of segmentation. These findings reveal a previously unrecognized role for Rac1-mediated signaling in coordinating the mechanical and molecular aspects of segmentation dynamics.