LPXRFa, the Piscine Ortholog of GnIH, and LPXRF Receptor Positively Regulate Gonadotropin Secretion in Tilapia (Oreochromis niloticus)

Abstract

LPXRFamide (LPXRFa) peptides have been characterized for their ability to inhibit gonadotropin (GTH) release in birds and stimulate growth hormone (GH) release in frogs. However, their involvement in regulating the reproductive hypothalamo-pituitary-gonadal axis in mammals and fish is inconclusive. To study the role of LPXRFa peptides in the regulation of GTH secretion, we cloned tilapia LPXRFa and LPXRF receptor (LPXRF-R). Processing of the tilapia preproLPXRFa liberated three mature LPXRFa peptides that varied in size and post-translational modifications. Phylogenetic analysis of LPXRFa and the closely related RFamide peptide PQRFa showed clear clustering of each peptide sequence with its orthologs from various vertebrates. Signal-transduction analysis of the tilapia LPXRF-R in COS-7 cells showed clear stimulation of CRE-dependent luciferase activity, whereas the human NPFFR1 showed suppression of forskolin-induced CRE-dependent activity in this system. Administration of the tilapia pyroglutaminated LPXRFa-2 peptide to primary cell culture of tilapia pituitaries, or to reproductive female tilapia by ip injection, positively regulated both LH and FSH release in vivo and in vitro. Using double-labeled fluorescent in-situ hybridization and immunofluorescence, βLH cells were found to co-express both tilapia lpxrf and tilapia lpxrf-r mRNA, whereas some of the βFSH cells coexpressed only lpxrf-r mRNA. No coexpression of tilapia lpxrf-r was identified in GH-positive cells. These findings suggest that the LPXRFa system is a potent positive regulator of the reproductive neuroendocrine axis of tilapia.

GnIH plays a negative role in regulating GtH expression in the common carp, Cyprinus carpio L.

“…Considering the changes in GnIH mRNA during reproduction, we concluded that GnIH plays a negative role in regulating GtH in the common carp. The results are consistent to those for the zebrafish and goldfish (Qi et al, 2013;Zhang et al, 2010), but opposite to the results obtained for the grass puffer, tilapia, and salmon (Amano et al, 2006;Biran et al, 2014;Shahjahan et al, 2011). The differing results may be related to the protein sequence of common carp GnIH compared with those of the goldfish, zebrafish, grass puffer, and tilapia.…”

“…In different species, the regulation of GnIH on GTH is resulted through different route. The influence of GnIH on GTH may be achieved by GnIHRs expressed on GTH-secreting cells (Biran et al, 2014;Maddineni et al, 2008).…”

“…So far, GnIH cDNA sequences have been identified in goldfish, grass puffer, zebrafish and tilapia (Biran et al, 2014;Shahjahan et al, 2011;Sawada et al, 2002;Zhang et al, 2010). In most of these reports, GnIH peptides were named LPXRFa-1/2/3.…”

“…In zebrafish, GnIH peptides were called GnIH-1/2/3 (Qi et al, 2013). However, GnIH peptides were called LPXRFa-1/2/3 because of the conserved LPXRF sequence at the C-terminal, but most peptides in tilapia and grass puffer do not contained the LPXRFa (Biran et al, 2014;Shahjahan et al, 2011).…”

“…In the grass puffer (Takifugu niphobles), incubation of pituitary cells with different doses of goldfish GnIH-I resulted in an increase in FSH and LH mRNA levels (Shahjahan et al, 2011). Administration of the GnIH-II peptide to tilapia pituitary cells or intraperitoneal injection of female tilapia resulted in an increase in FSH and LH proteins (Biran et al, 2014). It was also reported that regulation of LH secretion and gonadotropin subunit mRNA levels are dissociated in the presence of gGnIH and dependent on maturational status and administration route (Moussavi et al, 2012).…”