Identification of spliced mRNA isoforms of retinoid X receptor (RXR) in the Oriental freshwater prawn Macrobrachium nipponense

Retinoid X receptors (RXR) are members of the nuclear receptor family that are conserved from invertebrates to vertebrates, and they play an essential role in regulating reproductive maturation, molting, and embryo development. In this study, five RXR isoforms, named RXRL2 (L, long form), RXRL3, RXRS1 (S, short form), RXRS2, and RXRS3, containing six domains from A to F, were cloned from the prawn Macrobrachium nipponense using 5'- and 3'- rapid amplification of cDNA ends. Differences among their structures were observed not only in the D and E domains but also in the A/B domain, which were previously found in insects but not in crustaceans. This is the first report to show that differences occur in the A/B domain of RXR in crustaceans. RXR expressions were also examined in various tissues including the ovary, testis, muscle, hepatopancreas, heart, gill, stomach, intestine, and cuticle. Expression pattern investigations indicated that the five isoforms were differentially expressed. RXRS3 was only detected in the ovary, and the other RXRs were abundant in the ovary and testis. These data suggested that RXR mediates a series of processes related to reproduction.