Functional characterization of retinoid X receptor with an emphasis on the mediation of organotin poisoning in the Pacific oyster (Crassostrea gigas)

Marine mollusks suffer harmful effects due to environmental organotin compounds such as tributyltin (TBT) and triphenyltin (TPT). It is known that gastropod imposex caused by organotins is mediated by a key nuclear receptor, retinoid X receptor (RXR). The organotin-mediated toxic effects on oysters grown in seawater include a thicker shell, incomplete growth, disrupted development and a high rate of mortality. However, few studies have been conducted to determine the role of RXR in the toxic effects of organotins on bivalves. Here, we cloned an RXR homolog (CgRXR) from the Pacific oyster (Crassostrea gigas) and characterized its molecular function. Expression of the CgRXR RNA transcripts was assessed in whole developmental stages and tissues, with the highest expression detected in the blastula and mantle, respectively. The subcellular localization experiment confirmed that CgRXR protein was expressed in the nucleus exclusively as a nuclear receptor. Electrophoretic mobility shift assay indicated that CgRXR could bind to the DNA motifs DR0-DR5. The dual-luciferase reporter assay demonstrated that the transcriptional activity of CgRXR was activated by conserved ligands (9-cis retinoic acid and cis-4,7,10,13,16,19-docosahexanoic acid) and endocrine-disrupting chemicals (TBT and TPT). These results revealed the conserved gene function involved in protein localization, ligand binding and heterodimer formation with thyroid hormone receptor. However, the DNA binding properties of CgRXR differed from those of other invertebrate and vertebrate RXRs. CgRXR had the highest expression level in the blastula and mantle, and the disrupted development or shell malformation induced by organotins suggested a possible correlation of CgRXR with shell formation in bivalves. The results indicated the potential involvement of CgRXR in the toxic effects of organotins (TBT and TPT) through signaling pathway disruption. Functional characterization of CgRXR will help us better understand the endocrinology of bivalves.