Characterization of LPXRFa receptor in the half-smooth tongue sole (Cynoglossus semilaevis): Molecular cloning, expression profiles, and differential activation of signaling pathways by LPXRFa peptides.

Gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide, serves as a key player in the regulation of reproduction across vertebrates, acting on the brain and pituitary to modulate reproductive physiology and behavior. However, little information is available in teleosts regarding the intracellular signal transduction pathway in response to GnIH. To this end, we first cloned the gene of LPXRFa (the piscine ortholog of GnIH) receptor in the half-smooth tongue sole (Cynoglossus semilaevis), a representative species of the order Pleuronectiformes. The full-length cDNA of LPXRFa receptor was 2201 bp in size with an open reading frame (ORF) of 1365 bp that encoded 454 amino acids. Tissue distribution showed that LPXRFa receptor transcripts could be detected at high levels in the brain, to a lesser extent in the pituitary, and at low levels in the ovary and other peripheral tissues. In vitro functional analysis revealed that putative tongue sole LPXRFa-1 and LPXRFa-2 peptides significantly stimulated serum responsive element-dependent luciferase (SRE-luc) activity in COS-7 cells transfected with the novel receptor, and these stimulatory effects were evidently reduced by two inhibitors of the PLC/PKC pathway. In addition, neither LPXRFa-1 nor LPXRFa-2 altered the cAMP-responsive element (CRE)-luc activity, but only LPXRFa-2 could markedly decrease forskolin-induced CRE-luc activity in COS-7 cells expressing its cognate receptor. Taken together, our results encompass the first study reporting the existence of LPXRFa receptor in the order Pleuronectiformes and provide novel evidence of differential activation of signaling pathways by LPXRFa peptides in fish.