Abstract
Thyroid hormones are crucial regulators of metamorphosis and development in bilaterians, particularly in chordate deuterostomes. Recent evidence suggests a role for thyroid hormone signaling, principally via 3,5,3',5'-Tetraiodo-L-thyronine (T4), in the regulation of metamorphosis, programmed cell death and skeletogenesis in echinoids (sea urchins and sand dollars) and sea stars. Here we test whether TH signaling in skeletogenesis is a shared trait of Echinozoa (Echinoida and Holothouroida) and Asterozoa (Ophiourida and Asteroida). We demonstrate dramatic acceleration of skeletogenesis in three classes of echinoderms: sea urchins, sea stars, and brittle stars (echinoids, asteroids, and ophiuroids). Fluorescently labeled thyroid hormone analogues reveal thyroid hormone binding to cells proximal to regions of skeletogenesis in the gut and juvenile rudiment. Immunohistochemistry of phosphorylated MAPK in the presence and absence of TH binding inhibitors suggests that THs may act via phosphorylation of MAPK (ERK1/2) to accelerate skeletogenesis in the three echinoderm groups. Additionally, we detect thyroid hormone binding to the cell membrane and nucleus during metamorphic development in echinoderms. Together, these results indicate that TH regulation of mesenchyme cell activity via integrin-mediated MAPK signaling may be a conserved mechanism for the regulation of skeletogenesis in echinoderm development. Additionally, TH action via a nuclear thyroid hormone receptor may regulate metamorphic development. Our findings shed light on potentially ancient pathways of thyroid hormone activity in echinoids, ophiuroids, and asteroids, or on a signaling system that has been repeatedly co-opted to coordinate metamorphic development in bilaterians.