Department of Marine Molecular Biotechnology, College of Life Sciences, Gangneung-Wonju National University, Gangneung 210-702, Republic of Korea.
Gen Comp Endocrinol. 2012 Sep 1;178(2):380-90. doi: 10.1016/j.ygcen.2012.06.019. Epub 2012 Jun 23.
Retinoid X receptors (RXRs) are highly conserved members of the nuclear hormone receptor family that mediate various physiological processes in vertebrates and invertebrates. We examined the expression patterns of RXR in the ascidian Halocynthia roretzi across a wide range of tissues and stages of embryo development, as well as the regulation of gene transcription by the ascidian RXR. H. roretzi RXR cDNA (HrRXR) was cloned from 64-cell stage embryos. The overall amino acid sequence of HrRXR showed high sequence identity with a urochordate Ciona intestinalis RXR (58%), but the ligand-binding domain of HrRXR was more similar to vertebrate orthologs than to those of invertebrate RXRs. Based on a phylogenetic analysis, HrRXR belongs to a group of urochordates that are separate from vertebrate RXRs, showing a clear evolutionary history. Real-time quantitative polymerase chain reaction and whole-mount in situ hybridization analyses revealed that the HrRXR mRNA is of maternal origin during embryogenesis, and in the examined adult tissues it is expressed in the muscles, gills, gonads, and the hepatopancreas. Immunofluorescence and immunohistochemical staining demonstrated that HrRXR is localized to the nucleus and highly expressed in the gills and hepatopancreas. Unlike human RXRα, HrRXR did not show 9-cis retinoic acid- and bexarotene (LGD1069)-dependent transactivation. While a synthetic ligand for farnesoid X receptor (FXR), GW4064, did not increase the transcriptional activation in HrRXR- or HrRXR/HrFXR-transfected HEK-293 cells, the ligand showed weak but significant activity for a single amino acid mutant of HrRXR ((Phe)231(Cys)) and HrFXR cotransfected cells. The present study suggests that the marine invertebrate chordate RXR may possess endogenous ligands that are different than vertebrate RXR ligands and which function during early embryonic stages.
视黄酸 X 受体（RXR）是核激素受体家族中高度保守的成员，在脊椎动物和无脊椎动物中介导各种生理过程。我们研究了海鞘 Halocynthia roretzi 中 RXR 在广泛的组织和胚胎发育阶段的表达模式，以及海鞘 RXR 对基因转录的调节。从 64 细胞期胚胎中克隆了 H. roretzi RXR cDNA（HrRXR）。HrRXR 的整体氨基酸序列与尾索动物 Ciona intestinalis RXR 具有高度的序列同一性（58%），但 HrRXR 的配体结合域与脊椎动物同源物更相似，而与无脊椎动物 RXR 相似。基于系统发育分析，HrRXR 属于与脊椎动物 RXR 分开的尾索动物群，具有清晰的进化历史。实时定量聚合酶链反应和全胚胎原位杂交分析表明，HrRXR mRNA 在胚胎发生期间具有母体来源，在所检查的成年组织中，它在肌肉、鳃、性腺和肝胰腺中表达。免疫荧光和免疫组织化学染色表明，HrRXR 定位于细胞核，在鳃和肝胰腺中高度表达。与人类 RXRα 不同，HrRXR 不显示 9-顺式视黄酸和倍他罗汀（LGD1069）依赖性反式激活。虽然合成的法尼醇 X 受体（FXR）配体 GW4064 没有增加 HrRXR 或 HrRXR/HrFXR 转染的 HEK-293 细胞的转录激活，但该配体对 HrRXR 单一氨基酸突变体（Phe231Cys）和 HrFXR 共转染细胞显示出微弱但显著的活性。本研究表明，海洋无脊椎脊索动物 RXR 可能具有不同于脊椎动物 RXR 配体的内源性配体，并且在早期胚胎阶段发挥作用。
