Dorsal/ventral (DV) patterning of the sea urchin embryo relies on a ventrally-localized organizer expressing Nodal, a pivotal regulator of the DV gene regulatory network. However, the inceptive mechanisms imposing the symmetry-breaking are incompletely understood. In Paracentrotus lividus, the Hbox12 homeodomain-containing repressor is expressed by prospective dorsal cells, spatially facing and preceding the onset of nodal transcription. We report that Hbox12 misexpression provokes DV abnormalities, attenuating nodal and nodal-dependent transcription. Reciprocally, impairing hbox12 function disrupts DV polarity by allowing ectopic expression of nodal. Clonal loss-of-function, inflicted by blastomere transplantation or gene-transfer assays, highlights that DV polarization requires Hbox12 action in dorsal cells. Remarkably, the localized knock-down of nodal restores DV polarity of embryos lacking hbox12 function. Finally, we show that hbox12 is a dorsal-specific negative modulator of the p38-MAPK activity, which is required for nodal expression. Altogether, our results suggest that Hbox12 function is essential for proper positioning of the DV organizer.
Embryos begin as a collection of identical cells. As the embryo develops further, the cells in different regions must take on different structures and roles in order to form the complex tissues and organs seen in the fully developed organism. Therefore, a key task in early development is to inform cells where they are in a developing embryo. Signaling proteins released by special groups of organizing cells are responsible for providing the information about where a cell is located. Networks of genes controlled by these proteins then inform embryonic cells of where they are and what they should, or should not, become.
One such signaling protein is called Nodal, and is needed to perform a number of tasks in the developing embryo, including helping to form the basic tissues of the organism. Many animals depend on Nodal to develop correctly—from mice and humans, to zebrafish and sea urchins.
During sea urchin development, Nodal establishes where the mouth of a larva forms, setting up what is called the dorsal/ventral axis of the embryo; this separates the front and back of the embryo. To do so, the Nodal protein is mostly produced at the front of the embryo. Although much is already known about the network of genes that the Nodal protein controls, the genes and proteins that ensure that the initial source of Nodal is present at the right time and place are largely unknown.
Another protein called Hbox12 was also thought to be important for setting up the dorsal/ventral axis. Now, Cavalieri and Spinelli reveal that Hbox12 regulates Nodal during the development of a sea urchin embryo. In the early developing sea urchin, the gene that produces Hbox12 is activated in the region of the embryo that will become its back, directly opposite where Nodal is present. This activation normally occurs just before the gene that produces Nodal is turned on. If the hbox12 gene function is impaired, the Nodal protein is produced in both the front and the back sections of the embryo. Conversely, if Hbox12 is introduced into regions where Nodal is present, the amount of Nodal decreases. Furthermore, disrupting Hbox12 prevents any signs of the dorsal/ventral axis forming.
Cavalieri and Spinelli propose that Hbox12 inhibits the production of Nodal by briefly inactivating another protein that is required to activate the nodal gene. By doing so, Hbox12 sets up the dorsal/ventral axis by restricting Nodal to the cells that will make up the front half of the embryo.
Most complex organisms have asymmetric bodies, and failure to establish these body asymmetries can result in disease and other disorders in humans. Deciphering how the dorsal/ventral asymmetry in the sea urchin embryo is established should improve our understanding of how the mechanisms that form body shapes have evolved.