The evolutionary road to invertebrate thyroid hormone signaling: Perspectives for endocrine disruption processes

Nuclear Receptors and Development of Marine Invertebrates

“…THR is dynamically expressed during development of both mollusks and tunicates, which is suggestive of a possibly conserved function in bilaterian development ( Figure 5 ). THR genes originated at the base of bilaterians and have already been identified in the genomes of a wide variety of protostomes and deuterostomes [ 10 , 52 , 85 ]. However, the THR gene was lost in ecdysozoans and lineage-specific THR duplications occurred, for example, in platyhelminths, likely by independent duplication events in trematodes and turbellarians ( Figure 3 ) ( Table 2 ) [ 27 , 85 ].…”

“…However, in the absence of convincing evidence of endogenous TH synthesis in protostomes, it is currently believed that protostomes have to take up THs, or its precursors, from external sources [ 52 , 54 , 92 ]. In annelids, the THR of Platynereis dumerilii is activated in the presence of T3 or TRIAC [ 10 , 54 ]. Treatments with exogenous T3 or TRIAC induce an acceleration of the morphological switch from the trochophore to the crawling larva.…”

“…Treatments with exogenous T3 or TRIAC induce an acceleration of the morphological switch from the trochophore to the crawling larva. This morphological switch is further characterized by a peak of THR expression [ 10 , 54 ]. The THR of the annelid P. dumerilii might thus be a ligand-activated receptor with endogenous ligands similar to T3 or TRIAC that mediates developmental transitions between larval stages [ 54 , 93 ].…”

“…Efficient dimeric complexes are formed in vivo through cooperative protein-protein and protein-DNA interactions [ 7 ]. While protein-protein dimerization interfaces are found in the DBD, the LBD and the hinge region of NRs, REs serve as NR dimerization sites on DNA [ 8 , 9 , 10 ].…”

“…Even though it is still not clear to what extent the teratogenic effects of EDCs in invertebrates are mediated by NRs, it has been shown that NRs are involved in embryonic and post-embryonic development in these animals [ 47 , 48 , 49 , 50 , 51 , 52 , 53 ]. Yet, the mechanisms at play are not necessarily conserved with vertebrates, revealing lineage-specific adaptions in both invertebrates and vertebrates [ 1 , 10 , 17 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 ]. The aim of this review is to describe and discuss the developmental functions of NRs in invertebrates, with a special focus on marine organisms, highlighting the particular importance of comparative approaches using emerging marine invertebrate models for the field of evolutionary developmental biology.…”