The control of foxN2/3 expression in sea urchin embryos and its function in the skeletogenic gene regulatory network

Early development requires well-organized temporal and spatial regulation of transcription factors that are assembled into gene regulatory networks (GRNs). In the sea urchin, an endomesoderm GRN model explains much of the specification in the endoderm and mesoderm prior to gastrulation, yet some GRN connections remain incomplete. Here, we characterize FoxN2/3 in the primary mesenchyme cell (PMC) GRN state. Expression of foxN2/3 mRNA begins in micromeres at the hatched blastula stage and then is lost from micromeres at the mesenchyme blastula stage. foxN2/3 expression then shifts to the non-skeletogenic mesoderm and, later, to the endoderm. Here, we show that Pmar1, Ets1 and Tbr are necessary for activation of foxN2/3 in micromeres. The later endomesoderm expression of foxN2/3 is independent of the earlier expression of foxN2/3 in micromeres and is independent of signals from PMCs. FoxN2/3 is necessary for several steps in the formation of the larval skeleton. Early expression of genes for the skeletal matrix is dependent on FoxN2/3, but only until the mesenchyme blastula stage as foxN2/3 mRNA disappears from PMCs at that time and we assume that the protein is not abnormally long-lived. Knockdown of FoxN2/3 inhibits normal PMC ingression and foxN2/3 morphant PMCs do not organize in the blastocoel and fail to join the PMC syncytium. In addition, without FoxN2/3, the PMCs fail to repress the transfating of other mesodermal cells into the skeletogenic lineage. Thus, FoxN2/3 is necessary for normal ingression, for expression of several skeletal matrix genes, for preventing transfating and for fusion of the PMC syncytium.