Characterization of the somatostatin system in tilapia: implications for growth and reproduction.

Somatostatin (SST) plays diverse physiological roles in vertebrates, particularly in regulating growth hormone secretion from the pituitary. While the function of SST as a neuromodulator has been studied extensively, its role in fish and mammalian reproduction remains poorly understood. To address this gap, we investigated the involvement of the somatostatin system in the regulation of growth and reproductive hormones in tilapia. RNA sequencing of mature tilapia brain tissue revealed the presence of three SST peptides: SST6, SST3, and low levels of SST1. Four different isoforms of the somatostatin receptor (SSTR) subfamily were also identified in the tilapia genome. Phylogenetic and synteny analysis identified tiSSTR2-like as the root of the tree, forming two mega clades, with SSTR1 and SSTR4 in one and SSTR2a, SSTR3a, and SSTR5b in the other. Interestingly, the tiSSTR-5 isoforms 5x1, 5x2, and 5x3 were encoded in the sstr3b gene and were an artifact of misperception in the nomenclature in the database. RNA-seq of separated pituitary cell populations showed that SSTRs were expressed in gonadotrophs, with sstr3a enriched in luteinizing hormone (LH) cells and sstr3b significantly enriched in follicle-stimulating hormone (FSH) cells. Notably, cyclosomatostatin, an SSTR antagonist, induced cAMP activity in all SSTRs, with SSTR3a displaying the highest response, whereas octreotide, an SSTR agonist, showed a binding profile like that observed in human receptors. Binding site analysis of tiSSTRs from tilapia pituitary cells revealed the presence of canonical binding sites characteristic of peptide-binding class A G-protein-coupled receptors. Based on these findings, we explored the effect of somatostatin on gonadotropin release from the pituitary in vivo. Whereas cyclosomatostatin increased LH and FSH plasma levels at 2 h post-injection, octreotide decreased FSH levels after 2 h, but the LH levels remained unaffected. Overall, our findings provide important insights into the somatostatin system and its mechanisms of action, indicating a potential role in regulating growth and reproductive hormones. Further studies of the complex interplay between SST, its receptors, and reproductive hormones may advance reproductive control and management in cultured populations.

In silico insights into intra- and inter-species interactions of piscine gonadotropin hormones and receptor crosstalk.

In mammals, the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are macromolecules secreted during specific reproductive phases and display strict specificity towards their cognate receptors. However, fish gonadotropins (GTH) and their receptors (GTHR) display diverse species-specific expression patterns, secretion patterns, and intra- and interspecies cross-activation. To uncover the molecular basis of this diversity, we generated and analyzed 29 in-silico models of intra- and inter-species combinations of sturgeon, carp, tilapia, and human gonadotropins with piscine receptors and analyzed the resulting receptor activation and signal transduction of these GTHR-GTH complexes in-vitro. Our results suggest that unlike humans, the surface charge on piscine FSH/LH ß-seatbelt and N107huLHCGR/K104hFSHR homologs does not necessarily determine binding specificity. Instead, sequence and structural variations allow piscine GTHs significant conformational flexibility when binding to the receptor extracellular domain, thereby enabling cross-activation. The resulting diversity may support various reproductive strategies in different environmental niches.

Characteristics of Neurokinin-3 Receptor and Its Binding Sites by Mutational Analysis.

NKB (Neurokinin B) is already known to play a crucial role in fish reproduction, but little is known about the structure and function of NKB receptors. Based on an in silico model of the tilapia NKB receptor Tachykinin 3 receptor a (tiTac3Ra) found in the current study, we determined the key residues involved in binding to tilapia NKB and its functional homologue NKF (Neurokinin F). Despite studies in humans suggesting the crucial role of F2516.44 and M2897.43 in NKB binding, no direct peptide interaction was observed in tilapia homologs. In-silico, Ala mutations on residues F2516.44 and M2897.43 did not influence binding affinity, but significantly affected the stability of tiTac3Ra. Moreover, in vitro studies indicated them to be critical to tiNKB/tiNKF-induced receptor activity. The binding of NKB antagonists to tiTac3Ra both in-vitro and in vivo inhibits FSH (follicle stimulating hormone) and LH (luteinizing hormone) release and sperm production in mature tilapia males. Non-peptide NKB antagonist SB-222200 had a strong inhibitory effect on the Tac3Ra activation. SB-222200 also decreased LH plasma levels; two hours post intraperitoneal injection, changed sperm volume and the ratios of the different stages along the spermatogenesis in tilapia testes.

Molecular characterization of kisspeptin receptors and gene expression analysis during oogenesis in the Russian sturgeon (Acipenser gueldenstaedtii).

Sturgeons belong to a subclass of fishes that derived from ray-finned fish ancestors preceding the emergence of teleosts. The Russian sturgeon (Acipenser gueldenstaedtii) is a late-maturing fish with the females reaching puberty under aquaculture conditions at 6-10 years of age. Since kisspeptin has been shown to be a key hormone involved in regulation of major reproductive processes of many vertebrate species, this study was conducted to better understand the kisspeptin receptor (KissR) in sturgeon. In this study we have cloned Russian sturgeon KissR1 from brain mRNA and observed the ontogeny of rsKissR1 mRNA expression in ovarian follicles. Multiple sequence alignment of KissR1, KissR4, and their orthologs revealed that the Russian sturgeon (rs) KissR1 sequence shares 64%-77% identity with elephant shark, coelacanth, and gar and 44-58% identity with tetrapod and teleost KissR1 sequences, while KissR4 seemed to share <65% identity to eel KissR2 and ~57% identity to Perciformes and Cypriniformes. Further rsKissR4 showed <97% identity to reed fish KissR4, <63% with Squamata (Reptiles) and gar KissR4. A phylogenetic analysis revealed that rsKissR1 is more closely related to coelacanth and gar KissR1 than teleost, while rsKissR4 was part of the KissR4 clade and shared higher similarity with Actinopterygiian sequences. We have further predicted homology models for both rsKiss receptors and performed in-silico analyses of their binding to a kiss-10 peptide. Both sturgeon and zebrafish Kiss1 and Kiss2 activated rsKissR1 via both PKC/Ca2+ and PKA/cAMP signal-transduction pathways, while rsKissR2 was found to be less effective and was not activated by stKiss peptides. Ovarian rsKissR transcript levels for 10 fishes were determined by real-time PCR and significantly increased concomitantly with oogenesis, where the highest level of expression was evident in black follicles. These data suggest that extra-neuronal expression of the kisspeptin receptor may be involved in sturgeon reproduction in a manner dependent on reproductive development.

Molecular characterization of two Russian sturgeon gonadotropin receptors: Cloning, expression analysis, and functional activity.

Sturgeons are being used in aquaculture because wild populations are now endangered due to overfishing for caviar. A challenge in working with sturgeon as an aquacultured species is its long and slow reproductive development. Reproduction is a hormonally regulated process that involves hierarchical signaling between the brain, pituitary gland, and gonads. In an effort to better understand the hormonal regulation of sturgeon reproduction, we have cloned the Russian sturgeon (st), Acipenser gueldenstaedtii, luteinizing hormone receptor (stLHR) and follicle stimulating hormone receptor (stFSHR) and measured their expression from previtellogenic to mature ovarian follicles. Sturgeon LHR and FSHR expression was elevated in early-vitellogenic and mature follicles compared with pre-vitellogenic and mid-vitellogenic follicles, and only LHR expression increased during late-vitellogenesis. Recombinant sturgeon FSH and LH both activated sturgeon LHR and FSHR in a cAMP reporter assay. Further molecular characterization of these receptors was accomplished by in silico modeling and cAMP reporter assays using heterologous recombinant gonadotropins from human and piscine species. There was no apparent trend in heterologous LH and/or FSH activation of the sturgeon LHR or FSHR. These data suggest that permissive activation of LHR and FSHR are a consequence of some yet undetermined biological characteristic(s) of different piscine species.

Characterization of gonadotropin receptors Fshr and Lhr in Japanese medaka, Oryzias latipes.

Reproduction in vertebrates is controlled by the brain-pituitary-gonad axis, where the two gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) play vital parts by activating their cognate receptors in the gonads. The main purpose of this work was to study intra- and interspecies ligand promiscuity of teleost gonadotropin receptors, since teleost receptor specificity is unclear, in contrast to mammalian receptors. Receptor activation was investigated by transfecting COS-7 cells with either Fsh receptor (mdFshr, tiFshr) or Lh receptor (mdLhr, tiLhr), and tested for activation by recombinant homologous and heterologous ligands (mdFshßα, mdLhßα, tiFshßα, tiLhßα) from two representative fish orders, Japanese medaka (Oryzias latipes, Beloniformes) and Nile tilapia (Oreochromis niloticus, Cichliformes). Results showed that each gonadotropin preferentially activates its own cognate receptor. Cross-reactivity was detected to some extent as mdFshßα was able to activate the mdLhr, and mdLhßα the mdFshr. Medaka pituitary extract (MPE) stimulated CRE-LUC activity in COS-7 cells expressing mdlhr, but could not stimulate cells expressing mdfshr. Recombinant tiLhßα, tiFshßα and tilapia pituitary extract (TPE) could activate the mdLhr, suggesting cross-species reactivity for mdLhr. Cross-species reactivity was also detected for mdFshr due to activation by tiFshßα, tiLhßα, and TPE, as well as for tiFshr and tiLhr due to stimulation by mdFshßα, mdLhßα, and MPE. Tissue distribution analysis of gene expression revealed that medaka receptors, fshr and lhr, are highly expressed in both ovary and testis. High expression levels were found for lhr also in brain, while fshr was expressed at low levels. Both fshr and lhr mRNA levels increased significantly during testis development. Amino acid sequence alignment and three-dimensional modelling of ligands and receptors highlighted conserved beta sheet domains of both Fsh and Lh between Japanese medaka and Nile tilapia. It also showed a higher structural homology and similarity of transmembrane regions of Lhr between both species, in contrast to Fshr, possibly related to the substitution of the conserved cysteine residue in the transmembrane domain 6 in medaka Fshr with glycine. Taken together, this is the first characterization of medaka Fshr and Lhr using homologous ligands, enabling to better understand teleost hormone-receptor interactions and specificities. The data suggest partial ligand promiscuity and cross-species reactivity between gonadotropins and their receptors in medaka and tilapia.

Deciphering Direct and Indirect Effects of Neurokinin B and GnRH in the Brain-Pituitary Axis of Tilapia.

Neurokinin B (NKB) and its cognate receptor (NK3R) are emerging as important components of the neuroendocrine regulation of reproduction. Unlike mammalian tac3, which encodes only one mature peptide (namely NKB), two mature peptides are predicted for each tac3 gene in fish and frogs. Therefore, it was designated as Neurokinin F (NKF). Hormone analogs with high and long-lasting biological activity are important tools for physiological and biological research; however, the availability of piscine-specific analogs is very limited. Therefore, we have developed specific NKB and NKF analogs based on the structure of the mammalian NKB analog-senktide. These analogs, specifically designed for longer half-lives by methylation of proteolysis sites, exhibited activity equal to those of the native NKB and NKF in short-term signal-transduction assays of tilapia NKB receptors. However, the analogs were found to be able to significantly increase the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and growth hormone (GH) in tilapia, as fast as 1 h after intraperitoneal (IP) injection. The impact of the analogs on LH and FSH secretion lasted longer compared to the effect of native peptides and salmon GnRH analog (sGnRHa). In addition, we harvested pituitaries 24 h post injection and measured LH, FSH and GH mRNA synthesis. Both analogs elevated mRNA levels of LH and GH, but only NKB analog increased FSH mRNA levels in the pituitary and all GnRH forms in the brain. NKB receptors were co-localized with all three types the GnRH neurons in tilapia brain in situ. We previously showed a direct effect of NKB at the pituitary level, and these new results suggest that the stronger impact of the NKB analog on GTH release is also due to an indirect effect through the activation of GnRH neurons. These results suggest that novel synthetic NKB analogs may serve as a tool for both research and agricultural purposes. Finally, the biological activity and regulatory role of NKB in tilapia brain and pituitary suggest that the NKB/NKBR system in fish is an important reproductive regulator in a similar way to the kisspeptin system in mammals.