Nuclear hormone receptor signaling in amphioxus.

The nuclear hormone receptors (NRs) form a superfamily of transcription factors unified by conserved protein structure and mode of function. While most members of this superfamily are activated by ligands, such as thyroid hormones, steroids, vitamin D or retinoic acid, other NRs are called orphan receptors because they have no known ligand. NR-dependent signaling is crucial for vertebrate development with the majority of receptors being expressed in the developing embryo. Due to massive gene duplications during vertebrate diversification, there are usually more NRs in vertebrates than in invertebrates. In this study, we examine the evolutionary diversification of the NR superfamily and of NR-dependent signaling in chordates (vertebrates, tunicates, and amphioxus). We take advantage of the unique features of the genome of the invertebrate amphioxus, which is characterized by a vertebrate-like gene content without having undergone massive duplications, to assess the NR signaling complement (NRs and NR coregulators) of the ancestral chordate. We find 33 NRs in amphioxus, which are more NRs than originally anticipated. This increase is mainly due to an amphioxus-specific duplication of genes encoding receptors of the NR1H group. In addition, there are three heterologous NRs in amphioxus that could not be placed within the framework of the NR superfamily. Apart from these exceptions, there is usually one amphioxus NR or NR signaling coregulator for each paralogous group of two, three, or four human receptors suggesting that the ancestral chordate had a set of 22 different NRs plus one copy of each NR coregulator.