Novel Pituitary Actions of TAC3 Gene Products in Fish Model: Receptor Specificity and Signal Transduction for Prolactin and Somatolactin α Regulation by Neurokinin B (NKB) and NKB-Related Peptide in Carp Pituitary Cells

Abstract

TAC3 is a member of tachykinins, and its gene product neurokinin B (NKB) has recently emerged as a key regulator for LH through modulation of kisspeptin/GnRH system within the hypothalamus. In fish models, TAC3 not only encodes NKB but also a novel tachykinin-like peptide called NKB-related peptide (NKBRP), and the pituitary actions of these TAC3 gene products are still unknown. Using grass carp as a model, the direct effects and postreceptor signaling for the 2 TAC3 products were examined at the pituitary level. Grass carp TAC3 was cloned and confirmed to encode NKB and NKBRP similar to that of other fish species. In carp pituitary cells, NKB and NKBRP treatment did not affect LH release and gene expression but up-regulated prolactin (PRL) and somatolactin (SL)α secretion, protein production, and transcript expression. The stimulation by these 2 TAC3 gene products on PRL and SLα release and mRNA levels were mediated by pituitary NK2 and NK3 receptors, respectively. Apparently, NKB- and NKBRP-induced SLα secretion and transcript expression were caused by adenylate cyclase/cAMP/protein kinase A, phospholipase C/inositol 1,4,5-triphosphate/protein kinase C and Ca(2+)/calmodulin/Ca(2+)/calmodulin-dependent protein kinase II activation. The signal transduction for the corresponding responses on PRL release and mRNA expression were also similar, except that the protein kinase C component was not involved. These findings suggest that the 2 TAC3 gene products do not play a role in LH regulation at the pituitary level in carp species but may serve as novel stimulators for PRL and SLα synthesis and secretion via overlapping postreceptor signaling mechanisms coupled to NK2 and NK3 receptors, respectively.

Goldfish neurokinin B: Cloning, tissue distribution, and potential role in regulating reproduction

“…In addition, we provide the first evidence that NKBs up-regulate hormone gene expression in a fish model. A recent study in grass carp demonstrated that NKBs did not have effects on LH production in primary cultures of pituitary cell, but stimulated prolactin and somatolactin a synthesis and secretion from the pituitary (Hu et al, 2014). Thus, it is possible the NKBs regulate gonadotropin subunit mRNA expression indirectly via their effects on GnRH.…”

Novel Functional Role of NK3R Expression in the Potentiating Effects on Somatolactin α Autoregulation in grass carp pituitary cells

“…Given that (i) two somatolactin isoforms, SLα and SLβ, have been previously shown to trigger SLα secretion and gene expression at the pituitary level7, and (ii) TAC3a gene products could also stimulate SLα secretion and gene expression via activation of NK3R in carp pituitary cells14, we examined the functional interaction between TAC3a gene products and SLs in their stimulatory activity on SLα gene expression. As shown in Fig.…”

“…Based on the studies in mammals (e.g., rodent and sheep), NKB was found to be a key regulator for GnRH pulsatility and downstream LH release via NK3R activation in kisspeptin neurons located in the arcuate nuclei within the hypothalamus3031. Similar investigations have been recently extended to fish models with the novel findings that the TAC3 gene in fish species, e.g., in zebrafish323334, goldfish35, tilapia36 and grass carp14, not only encodes NKB as the gene product, but also the mature peptide of a new member of tachykinin called NKB-related peptide (NKBRP). Similar to NKB, NKBRP was also effective in stimulating LH release, e.g., in zebrafish32, goldfish35 and tilapia36, suggesting the reproductive function of TAC3 gene products was well conserved throughout vertebrate evolution.…”

“…To date, SL has been shown to be involved in diverse functions in fish models, including chromatophore proliferation and differentiation6, pigment aggregation7, inflation of swim bladder during embryo development5, reproduction3, stress responses8, lipid metabolism9, and osmoregulation10. At the pituitary level, somatolactin secretion and gene expression are known to be under the control by various hypothalamic factors, e.g., dopamine11, GnRH1213, PACAP4, neurokinin B (NKB)14, MCH15 and octadecdaneuropeptide16. Recently, insulin-like growth factors (IGFs) have also been identified as potent stimulators for SLα and SLβ secretion and gene expression in grass carp pituitary cells17.…”

“…Similar to NKB, NKBRP was also effective in stimulating LH release, e.g., in zebrafish32, goldfish35 and tilapia36, suggesting the reproductive function of TAC3 gene products was well conserved throughout vertebrate evolution. Based on our recent in vitro studies in grass carp pituitary cells, interestingly, NKB and NKBRP were found to have no effect on LH secretion and LHβ & GtHα gene expression at the pituitary level but rather serve as novel stimulators for prolactin (PRL) and SLα secretion and gene expression via differential activation of NK2R and NK3R expressed in the carp pituitary14. The investigation on SLα regulation by TAC3 gene products in the carp model have become more exciting with the recent demonstration that IGF-I/-II could act in a synergistic manner with TAC3 gene products, namely NKB and NKBRP, to up-regulate SLα gene expression at the pituitary level and this potentiating effect could be paralleled with the concurrent rise in NK3R expression induced by IGF-I/-II treatment.…”