Molecular characterization of the prolactin receptor in two fish species, tilapia Oreochromis niloticus and rainbow trout, Oncorhynchus mykiss: A comparative approach

We present recent information on the molecular characterization of the prol actin receptor (PRL-R) in two teleost species, tilapia (Oreochromis nilotic us) and rainbow trout (Oncorhynchus mykiss), in the perspective of improved understanding of the physiological differences in the control of osmoregul atory function between these two fish species. Although our interest will m ainly focus on osmoregulatory organs, we will also discuss evidence of the presence of PRL-R in other tissues such as gonads and hematopoietic organs. The first fish PRL-R was characterized in tilapia. This receptor is simila r to that of the long form of mammalian PRL-R, but the most conserved regio n (extracellular domain) has only 53% identity with mammalian PRL-R. A rain bow trout PRL-R cDNA has been also isolated and appeared very similar in st ructure to tilapia PRL-R. Expression of the PRL-R gene was studied by North ern blotting for various tissues from tilapia and trout, and a unique trans cript size of 3.2-3.4 kb was observed in all tissues studied (including mal e and female gonads, skin, brain, spleen, head, kidney, and circulating lym phocytes). Osmoregulatory organs (gills, kidney, intestine) were the riches t tissues. Using in situ hybridization, PRL-R transcripts were localized in gill chloride cells, both in trout and tilapia. Analysis of PRL-R transcri pt levels in gills, kidney, and intestine indicated the maintenance of a hi gh level of expression during adaptation to a hyperosmotic environment. The se results support PRL being a pleiotropic hormone in fish and suggest the presence of a unique PRL-R form in tilapia and in trout. Finally, character ization of hormone receptor binding has been carried out in both species us ing a radioreceptor assay (in tilapia) or surface plasmon resonance (SPR) t echnology (in trout). These studies indicated the presence of a stable horm one-receptor complex in tilapia, while PRL binds to its receptor through an unstable homodimeric complex in trout. Thus, the characteristics of PRL bi nding on its receptor appear to be significantly different in tilapia and t rout. Whether such differences may lead to different signal transduction me chanisms and osmoregulatory actions of PRL in these two euryhaline species merits further investigation.