Cell fate refinement during morphogenesis is regulated by an interplay between gene regulatory networks and the actomyosin network

During embryo development, cell interactions with the extracellular environment provides mechanical cues, necessary for the refinement of cell fate specification and morphogenesis. The sea urchin larval skeletogenesis is an excellent system to investigate the interplay between the gene regulatory network (GRN) and the protein networks that sense the mechanical properties of the environment and respond to it. The sea urchin calcite spicules are formed within a tubular compartment generated by the skeletogenic cells. The sea urchin skeletogenic gene regulatory network is known in great details and apparently evolved from an ancestral tubulogenesis program from which vertebrates’ vascularization had evolved. In the early stages of sea urchin skeletogenesis, skeletogenic genes are uniformly expressed in the skeletogenic lineage. However, when the spicules form the skeletogenic cells become more specialized and distinct regulatory states control the elongation of individual skeletal rods. This differential gene expression is also observed is skeletogenic cell cultures suggesting that the skeletogenic cells are able to autonomically distinguish between the tips and the back of the skeletal rods. Here we show that actomyosin remodeling and specifically, the Rho-associated coiled-coil kinase (ROCK), regulates skeletal formation, elongation and gene expression in the sea urchin embryo. ROCK inhibition in whole embryo and in skeletogenic cell cultures significantly delays skeletal growth, induces sever ectopic branching and prevents the formation of tip-specific regulatory states. We propose that ROCK and the actomyosin network, sense the spicule stiffness and transde it into gene expression, providing a necessary feedback between spicule elongation and the skeletogenic GRN.