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.

中文翻译：

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两栖动物中的核激素受体信号传导。

核激素受体（NRs）形成由保守的蛋白质结构和功能模式统一的转录因子超家族。尽管该超家族的大多数成员都被配体激活，例如甲状腺激素，类固醇，维生素D或视黄酸，但其他NR被称为孤儿受体，因为它们没有已知的配体。NR依赖性信号对于脊椎动物的发育至关重要，因为大多数受体都在发育中的胚胎中表达。由于脊椎动物多样化过程中大量的基因复制，因此脊椎动物中的NR通常比无脊椎动物中的NR更多。在这项研究中，我们检查了NR超家族的进化多样性以及和弦（椎骨，被膜和两栖动物）中NR依赖的信号传导。我们利用无脊椎动物文昌鱼的基因组的独特特征，其特征是无需进行大量重复就可以鉴定脊椎动物样基因的含量，以评估祖传cho酸盐的NR信号补体（NR和NR共调节剂）。我们在两栖动物中发现了33个NR，比原先的预期要多。这种增加主要是由于编码NR1H组受体的基因的两栖类特异性复制。此外，两栖类中有3个异源NR无法放置在NR超家族的框架内。除了这些例外，对于两个，三个或四个人类受体的每个旁系同源群，通常有一个两栖类NR或NR信号转导调节剂，这表明祖传的r酸盐具有22种不同的NRs加上每个NR调节剂的一个拷贝。