TAC1 Gene Products Regulate Pituitary Hormone Secretion and Gene Expression in Prepubertal Grass Carp Pituitary Cells

The regulation of prolactin secretion and its targeting function of teleost

Prolactin is involved in regulating various physiological activities of vertebrates and is one of the most momentous pituitary hormones. However, not enough attention is currently paid to prolactin, especially in teleost. This paper aims to gather, organize, and analyze recent studies on the regulation and functions of prolactin. By comparing with other animal groups, it highlights the significant role of prolactin in fish reproduction, immunity, growth, and osmotic pressure regulation, as well as the upstream and downstream factors that may be involved in the regulation of prolactin functions were introduced to provide a theoretical basis for the in-depth study and potential practical application of prolactin.

IGFBP1a is a nutrient deficient response factor that can inhibit fish reproduction through the hypothalamus-pituitary-ovary axis†

Reproduction is a high energy consuming process, so long-term malnutrition can significantly inhibit gonadal development. However, little is known about the molecular mechanism by which fasting inhibits reproduction. Our present study found that fasting could dramatically induce insulin-like growth factor binding protein 1 (IGFBP1) expression in the liver, hypothalamus, pituitary and ovaries of grass carp. In addition, IGFBP1a in the hypothalamus-pituitary-gonad axis could inhibit the development of gonads. These results indicated that fasting may participate in the regulation of fish gonadal development through the mediation of IGFBP1a. Further studies found that IGFBP1a could markedly inhibit gonadotropin-releasing hormone 3 expressions in hypothalamus cells. At the pituitary level, IGFBP1a could significantly reduce the gonadotropin hormones (LH and FSH) expression by blocking the action of pituitary insulin-like growth factor 1. Interestingly, IGFBP1a could also directly inhibit the expression of lhr, fshr, and sex steroid hormone synthase genes (cyp11a, cyp17a, and cyp19a1) in the ovary. These results indicated that IGFBP1a should be a nutrient deficient response factor that could inhibit fish reproduction through the hypothalamus-pituitary-ovary axis.

The Effects of Tachykinin1 Gene Products on Prepubertal Dabry's Sturgeon (Acipenser dabrynus) Pituitary Hormone Secretion and Gene Expression

As an ancient and endangered species unique to the Yangtze River in China, the wild population of the Dabry's sturgeon has become scarce. Due to the long time till the first sexual maturity of Dabry's sturgeon, the population of artificially bred Dabry's sturgeon recovered slowly. As a member of the tachykinin family, TAC1 has been reported to have a variety of functions in mammals such as pain control, smooth muscle contraction and reproductive cycle regulation, but the function of Tac1 in fish has been rarely reported. In this study, we synthesized two tac1 gene products, Substance P (SP) and neurokinin A (NKA), and further verified the effect of two tac1 gene products on the secretion of related hormones in the pituitary of Dabry's Sturgeon by intraperitoneal injection and co-culture of primary cells. Expression studies revealed that the newly cloned tac1 were mainly distributed in the hypothalamus and pituitary tissue of the brain. In prepubertal Dabry's sturgeon, this study showed that the two gonadotropins' mRNA levels in pituitary tissue can be significantly increased by SP and NKA through intraperitoneal injection, and the LH protein level in serum was also increased. Further study showed that both NKA and SP could promote the two gonadotropins' mRNA expression in pituitary cells of Dabry's sturgeon. In addition, we explored the optimal dose and time of SP and NKA on pituitary cells is 24 h and over 10 nM. These results, as a whole, suggested that tac1 gene products play an important role in gonadotropin release and gonadal development in prepubertal Dabry's sturgeon.

Sequences analysis and pituitary actions of tachykinins in Chinese sturgeon (Acipenser sinensis)

Tachykinins belong to a large, evolutionarily conserved family of brain/gut peptides that are involved in a variety of physiological functions in mammals, such as reproductive regulation. However, little information was available about tachykinins in ancient fish lineage. In the present study, we firstly identified three tachykinin genes (named tac1, tac3 and tac4) and three neurokinin receptors (named nk1r, nk2r and nk3r) from Chinese sturgeon brain and pituitary. Sequence analysis showed that tac1 encoded substance P (SP) and neurokinin A (NKA), tac3 encoded neurokinin B (NKB) and NKB-related peptide (NKBRP), and tac4 encoded hemokin 1 (HK-1) and hemokin 2 (HK-2), respectively. The luciferase reporter assay results showed that NK1R preferentially selected asSP, NK2R preferentially selected asNKA, and NK3R preferentially selected asNKB. Tissue expression analysis showed that the three tac genes were highly detected in the telencephalon and hypothalamus, whereas nkr genes were widely expressed in peripheral tissues. Spatio-temporal expression analysis showed that all three tac genes were highly expressed in unknown sex individuals. Intraperitoneal injection experiments showed that both asSP and asNKB could stimulate luteinizing hormone (LH) release in Chinese sturgeon serum. At the transcriptional level, asSP and asNKB could significantly reduce pituitary follicle-stimulating hormone beta (fshβ) mRNA expression, but induce pituitary growth hormone (gh) mRNA expression. In addition, estradiol (E2) could stimulate tac3 mRNA expression in hypothalamus. Taken together, this study provided information on the tachykinin family in Chinese sturgeon and demonstrates that asNKB and asSP could be involved in reproductive and growth regulation in pituitary.

SP protects Nile tilapia (Oreochromis niloticus) against acute Streptococcus agalatiae infection

Substance P (SP) is a neuropeptide that involves in a wide variety of physiological and pathological events, mainly exerts its roles by neurokinin 1 receptor (NK1R), also modulates immune function. However, the roles of SP during immune response to acute bacterial infection of Nile tilapia (Oreochromis niloticus) remain unclear. In this study, the gene of SP precursor (tachykinin precursor 1, TAC1) and the gene of SP receptor (NK1R) from Nile tilapia were identified, and the roles of SP during an acute bacterial infection in a warm water environment were investigated. On-TAC1(Oreochromis niloticus-TAC1) contains conservative SP & NKA peptide sequences and On-NK1R contains seven conservative transmembrane domains. Their transcriptional levels were most abundant in brain and the On-TAC1 transcripts can be induced in the tilapia challenged with Streptococcus agalactiae. Furthermore, the experimental results revealed that On-SP could promote pyroptosis, suppress inflammation, and improve survival rate during acute bacterial infection. The present data lays a theoretical foundation to further elucidate the mechanism of SP protecting fish against pathogens.

Endoscopic vs. microscopic transsphenoidal surgery outcomes in 514 nonfunctioning pituitary adenoma cases

Transsphenoidal surgery remains the preference choice for patients with nonfunctioning pituitary tumors at present, but the superiority of surgical modalities is still debated. Moreover, the efficacy of microscopic and endoscopic transsphenoidal surgery has not been fully studied. Therefore, the present study was designed to compare the postoperative outcomes of the two widely used approaches worldwide. This retrospective study included 514 patients with the first transsphenoidal pituitary adenoma resection in Shandong Provincial Hospital from January 2015 to July 2020 and compared the outcomes of microscope transsphenoidal surgery (MTSS) and endoscopy transsphenoidal surgery (ETSS). A total of 514 patients were included in this study, of whom 210 received the ETSS and 304 received the MTSS. The patients in two groups were similar in terms of postoperative hyponatremia (p = 0.229), diabetes insipidus (p = 0.264), the recovery of hormonal axis (p < 0.05), and extent of resection (EOR) (p = 0.067). ETSS was more likely to cause cerebrospinal fluid leakage than MTSS (p = 0.017, 3.6% vs. 8.6%). CSF leakage might be related to tumor size (95% CI = 1.305-2.766, p = 0.001), and the surgeon's transsphenoidal surgery volume < 300 was also a risk factor (95% CI = 1.396-9.067, p = 0.008). The effect of different surgeries on postoperative vision improvement was statistically difference in univariate analysis (p = 0.048) but not after adjustment for confounders (p = 0.112). Furthermore, there were statistical difference in EOR between MTSS and ETSS when adenomas were performed suprasellar extension (p = 0.037) or optic chiasm compression (p = 0.045). Both techniques are valid for the treatment of nonfunctional adenomas. But CSF leakage is more likely after ETSS. In addition, ETSS is more conducive to resection of nonfunctional adenomas with suprasellar extension or optic chiasm compression.

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

Novel pituitary actions of GnRH in teleost: The link between reproduction and feeding regulation

Gonadotropin-releasing hormone (GnRH), as a vital hypothalamic neuropeptide, was a key regulator for pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the vertebrate. However, little is known about the other pituitary actions of GnRH in teleost. In the present study, two GnRH variants (namely, GnRH2 and GnRH3) and four GnRH receptors (namely, GnRHR1, GnRHR2, GnRHR3, and GnRHR4) had been isolated from grass carp. Tissue distribution displayed that GnRHR4 was more highly detected in the pituitary than the other three GnRHRs. Interestingly, ligand-receptor selectivity showed that GnRHR4 displayed a similar and high binding affinity for grass carp GnRH2 and GnRH3. Using primary culture grass carp pituitary cells as model, we found that both GnRH2 and GnRH3 could not only significantly induce pituitary reproductive hormone gene (GtHα, LHβ, FSHβ, INHBa, secretogranin-2) mRNA expression mediated by AC/PKA, PLC/IP₃/PKC, and Ca²⁺/CaM/CaMK-II pathways but also reduce dopamine receptor 2 (DRD2) mRNA expression via the Ca²⁺/CaM/CaMK-II pathway. Interestingly, GnRH2 and GnRH3 could also stimulate anorexigenic peptide (POMCb, CART2, UTS1, NMBa, and NMBb) mRNA expression via AC/PKA, PLC/IP₃/PKC, and Ca²⁺/CaM/CaMK-II pathways in grass carp pituitary cells. In addition, food intake could significantly induce brain GnRH2 mRNA expression. These results indicated that GnRH should be the coupling factor to integrate the feeding metabolism and reproduction in teleost.

Multiple tachykinins and their receptors characterized in the gastropod mollusk Pacific abalone: Expression, signaling cascades, and potential role in regulating lipid metabolism

Tachykinin (TK) families, including the first neuropeptide substance P, have been intensively explored in bilaterians. Knowledge of signaling of TK receptors (TKRs) has enabled the comprehension of diverse physiological processes. However, TK signaling systems are largely unknown in Lophotrochozoa. This study identified two TK precursors and two TKR isoforms in the Pacific abalone Haliotis discus hannai (Hdh), and characterized Hdh-TK signaling. Hdh-TK peptides harbored protostomian TK-specific FXGXRamide or unique YXGXRamide motifs at the C-termini. A phylogenetic analysis showed that lophotrochozoan TKRs, including Hdh-TKRs, form a monophyletic group distinct from arthropod TKRs and natalisin receptor groups. Although reporter assays demonstrated that all examined Hdh-TK peptides activate intracellular cAMP accumulation and Ca²⁺ mobilization in Hdh-TKR-expressing mammalian cells, Hdh-TK peptides with N-terminal aromatic residues and C-terminal FXGXRamide motifs were more active than shorter or less aromatic Hdh-TK peptides with a C-terminal YXGXRamide. In addition, we showed that ligand-stimulated Hdh-TKRs mediate ERK1/2 phosphorylation in HEK293 cells and that ERK1/2 phosphorylation is inhibited by PKA and PKC inhibitors. In three-dimensional in silico Hdh-TKR binding modeling, higher docking scores of Hdh-TK peptides were consistent with the lower EC50 values in the reporter assays. The transcripts for Hdh-TK precursors and Hdh-TKR were highly expressed in the neural ganglia, with lower expression levels in peripheral tissues. When abalone were starved for 3 weeks, Hdh-TK1 transcript levels, but not Hdh-TK2, were increased in the cerebral ganglia (CG), intestine, and hepatopancreas, contrasting with the decreased lipid content and transcript levels of sterol regulatory element-binding protein (SREBP). At 24 h post-injection in vivo, the lower dose of Hdh-TK1 mixture increased SREBP transcript levels in the CG and hepatopancreas and accumulative food consumption of abalone. Higher doses of Hdh-TK1 and Hdh-TK2 mixtures decreased the SREBP levels in the CG. When Hdh-TK2-specific siRNA was injected into abalone, intestinal SREBP levels were significantly increased, whereas administration of both Hdh-TK1 and Hdh-TK2 siRNA led to decreased SREBP expression in the CG. Collectively, our results demonstrate the first TK signaling system in gastropod mollusks and suggest a possible role for TK peptides in regulating lipid metabolism in the neural and peripheral tissues of abalone.

Novel Pituitary Actions of TAC4 Gene Products in Teleost

Tachykinin 4 (TAC4) is the latest member of the tachykinin family involved in several physiological functions in mammals. However, little information is available about TAC4 in teleost. In the present study, we firstly isolated TAC4 and six neurokinin receptors (NKRs) from grass carp brain and pituitary. Sequence analysis showed that grass carp TAC4 could encode two mature peptides (namely hemokinin 1 (HK1) and hemokinin 2 (HK2)), in which HK2 retained the typical FXGLM motif in C-terminal of tachyinin, while HK1 contained a mutant VFGLM motif. The ligand-receptor selectivity showed that HK2 could activate all 6 NKRs but with the highest activity for the neurokinin receptor 2 (NK2R). Interestingly, HK1 displayed a very weak activation for each NKR isoform. In grass carp pituitary cells, HK2 could induce prolactin (PRL), somatolactin α (SLα), urotensin 1 (UTS1), neuromedin-B 1 (NMB1), cocaine- and amphetamine-regulated transcript 2 (CART2) mRNA expression mediated by NK2R and neurokinin receptor 3 (NK3R) via activation cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA), phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP3)/protein kinase C (PKC) and calcium²⁺ (Ca²⁺)/calmodulin (CaM)/calmodulin kinase-II (CaMK II) cascades. However, the corresponding stimulatory effects triggered by HK1 were found to be notably weaker. Furthermore, based on the structural base for HK1, our data suggested that a phenylalanine (F) to valine (V) substitution in the signature motif of HK1 might have contributed to its weak agonistic actions on NKRs and pituitary genes regulation.

Different pituitary action of NK3Ra and NK3Rb in grass carp

In mammals, NK3R is the specific receptor for NKB, which played an important role in reproduction. Recently, two NK3R isoforms, namely NK3Ra and NK3Rb, have been identified in fish. However, little is known about the pituitary actions of the two NK3R isoforms in fish. In this study, both NK3Ra and NK3Rb were isolated from grass carp pituitary. Although their sequence similarity was only 61.6%, the two NK3R isoforms displayed similar ligand selectivity and binding affinity to TAC3 gene products (NKBa, NKBRPa and NKBRPb). In addition, both NK3Ra and NK3Rb displayed similar signaling pathways, including PKA, PKC, MAPK and Ca²⁺ cascades. Tissue distribution indicated that both NK3Ra and NK3Rb were highly detected in grass carp pituitary. Further study found that NK3Ra was mainly located in pituitary LHβ cells, while NK3Rb was only detected in pituitary SLα cells. Furthermore, NK3Ra and NK3Rb activation could induce LHβ and SLα promoter activity, respectively. These results suggested that the two NK3R isoforms displayed different pituitary actions in fish. Using grass carp pituitary cells as model, we found that PACAP could significantly reduce NK3Ra, but induce NK3Rb mRNA expression coupled with cAMP/PKA and PLC/PKC pathways. Interestingly, PACAP could also significantly inhibit LHβ, but stimulate SLα mRNA expression in grass carp pituitary cells. Furthermore, NK3R antagonist could not only inhibit LHβ mRNA expression, but also block PACAP-induced SLα mRNA expression in grass carp pituitary cells. These results suggested that NK3Ra and NK3Rb could mediate PACAP-reduced LHβ and -induced SLα mRNA expression in grass carp pituitary, respectively.

Mouse Spexin: (III) Differential Regulation by Glucose and Insulin in Glandular Stomach and Functional Implication in Feeding Control

Spexin (SPX), a neuropeptide with diverse functions, is a novel satiety factor in fish models and its role in feeding control has been recently confirmed in mammals. In mouse, food intake was shown to trigger SPX expression in glandular stomach with parallel rise in serum SPX and these SPX signals could inhibit feeding via central actions within the hypothalamus. However, the mechanisms for SPX regulation by food intake are still unclear. To examine the role of insulin signal caused by glucose uptake in SPX regulation, the mice were IP injected with glucose and insulin, respectively. In this case, serum SPX was elevated by glucose but not altered by insulin. Meanwhile, SPX transcript expression in the glandular stomach was up-regulated by glucose but the opposite was true for insulin treatment. Using in situ hybridization, the differential effects on SPX gene expression were located in the gastric mucosa of glandular stomach. Co-injection experiments also revealed that glucose stimulation on serum SPX and SPX mRNA expressed in glandular stomach could be blocked by insulin. In gastric mucosal cells prepared from glandular stomach, the opposite effects on SPX transcript expression by glucose and insulin could still be noted with similar blockade of the stimulatory effects of glucose by insulin. In this cell model, SPX gene expression induced by glucose was mediated by glucose uptake via GLUT, ATP synthesis by glycolysis/respiratory chain, and subsequent modulation of KATP channel activity, but the voltage-sensitive Ca2+ channels were not involved. The corresponding inhibition by insulin, however, was mediated by PI3K/Akt, MEK1/2/ERK1/2, and P38MAPK cascades coupled to insulin receptor but not IGF-1 receptor. Apparently, glucose uptake in mice can induce SPX expression in the glandular stomach through ATP synthesis via glucose metabolism and subsequent modification of KATP channel activity, which may contribute to SPX release into circulation to act as the satiety signal after food intake. The insulin rise caused by glucose uptake, presumably originated from the pancreas, may serve as a negative feedback to inhibit the SPX response by activating MAPK and PI3K/Akt pathways in the stomach.

Reproductive Regulation of PrRPs in Teleost: The Link Between Feeding and Reproduction

Prolactin-releasing peptide (PrRP), a sort of vital hypothalamic neuropeptide, has been found to exert an enormous function on the food intake of mammals. However, little is known about the functional role of PrRP in teleost. In the present study, two PrRP isoforms and four PrRP receptors were isolated from grass carp. Ligand-receptor selectivity displayed that PrRP1 preferentially binds with PrRP-R1a and PrRP-R1b, while PrRP-R2a and PrRP-R2b were special receptors for PrRP2. Tissue distribution indicated that both PrRPs and PrRP-Rs were highly expressed in the hypothalamus-pituitary-gonad axis and intestine, suggesting a latent function on food intake and reproduction. Using grass carp as a model, we found that food intake could significantly induce hypothalamus PrRP mRNA expression, which suggested that PrRP should be also an anorexigenic peptide in teleost. Interestingly, intraperitoneal (IP) injection of PrRPs could significantly induce serum luteinizing hormone (LH) secretion and pituitary LHβ and GtHα mRNA expression in grass carp. Moreover, using primary culture grass carp pituitary cells as a model, we further found that PrRPs could directly induce pituitary LH secretion and synthesis mediated by AC/PKA, PLC/IP3/PKC, and Ca2+/CaM/CaMK-II pathways. Finally, estrogen treatment of prepubertal fish elicited increases in PrRPs and PrPR receptors expression in primary cultured grass carp hypothalamus cells, which further confirmed that the PrRP/PrRPR system may participate in the neuroendocrine control of fish reproduction. These results, taken together, suggest that PrRPs might act as a coupling factor in feeding metabolism and reproductive activities in teleost.

Comparative transcriptome analyses and identification of candidate genes involved in vertebral abnormality of bighead carp Hypophthalmichthys nobilis

Body deformity occurs both in wild and farmed fishes, which is one of the most challenging problems for aquaculture industry. In most cases, such body deformities are linked to skeletal deformities. Currently, very limited information is available on skeletal deformities of farmed fish species which may be caused by genetic factor. In this study, we performed muscle and vertebra transcriptome analyses in body deformity and normality of bighead carp Hypophthalmichthys nobilis (from one meiotic gynogenesis family) using RNA-Seq. A total of 43,923 and 44,416 unigenes were predicted in muscles and vertebrae, respectively. Based on these data, we further explored the gene expression profiles in gynogenetic normal and abnormal bighead carp. No differentially expressed gene (DEG) was found in transcriptome data of muscles. Totally, 20 key DEGs were identified in transcriptome data of vertebrae, such as low density lipoprotein-related protein 2 (lrp2), bone morphogenetic protein 2B (bmp2b) and collagen alpha-1(IV) (col4a1). 12 potential pathways were also identified in vertebra transcriptome data, which were mainly involved in development, growth, cytoskeleton and energy metabolism, such as MAPK signaling pathway, regulation of actin cytoskeleton and TGF-beta signaling pathway. Results of this study will be informative for the understanding of genetic mechanisms for body shape formation and also provide potential candidate genes for selection program involved in body shape and skeletal development in H. nobilis.

The role of neurokinin A and its receptor in the regulation of prolactin secretion by the anterior pituitary of cyclic pigs

In pigs, plasma prolactin concentration markedly changes during the oestrous cycle and the regulation of its secretion is very complex. The contribution of neurokinins in this process has not been sufficiently delineated. The aim of the study was to examine the effects of neurokinin A (NKA) on prolactin synthesis and secretion in cyclic gilts. The expression of NKA precursor (Ppta) and receptor (Tacr2) genes as well as NKA and TACR proteins content in the porcine pituitaries (days 2-3, 9-10, 12-13, 15-16 and 19-20 of the cycle) was determined. Furthermore, the in vitro influence of NKA on the expression of prolactin (Prl), dopamine receptor (D2r), TRH receptor (Trhr) genes and prolactin secretion by the porcine pituitary cells (days 9-10, 15-16 and 19-20 of the cycle) was assessed. The expression of Ppta and Tacr2 as well as NKA and TACR proteins in the pituitary tissue has been changing throughout the oestrous cycle. NKA affected in vitro the expression of studied genes and prolactin secretion depending on the stage of the cycle, dose of NKA and/or duration of the cell incubation. Altogether, the study indicates that NKA is engaged in the modulation of prolactin secretion in the pig during the oestrous cycle.

Tachykinins: Neuropeptides That Are Ancient, Diverse, Widespread and Functionally Pleiotropic

Tachykinins (TKs) are ancient neuropeptides present throughout the bilaterians and are, with some exceptions, characterized by a conserved FX₁GX₂Ramide carboxy terminus among protostomes and FXGLMamide in deuterostomes. The best-known TK is the vertebrate substance P, which in mammals, together with other TKs, has been implicated in health and disease with important roles in pain, inflammation, cancer, depressive disorder, immune system, gut function, hematopoiesis, sensory processing, and hormone regulation. The invertebrate TKs are also known to have multiple functions in the central nervous system and intestine and these have been investigated in more detail in the fly Drosophila and some other arthropods. Here, we review the protostome and deuterostome organization and evolution of TK precursors, peptides and their receptors, as well as their functions, which appear to be partly conserved across Bilateria. We also outline the distribution of TKs in the brains of representative organisms. In Drosophila, recent studies have revealed roles of TKs in early olfactory processing, neuromodulation in circuits controlling locomotion and food search, nociception, aggression, metabolic stress, and hormone release. TK signaling also regulates lipid metabolism in the Drosophila intestine. In crustaceans, TK is an important neuromodulator in rhythm-generating motor circuits in the stomatogastric nervous system and a presynaptic modulator of photoreceptor cells. Several additional functional roles of invertebrate TKs can be inferred from their distribution in various brain circuits. In addition, there are a few interesting cases where invertebrate TKs are injected into prey animals as vasodilators from salivary glands or paralyzing agents from venom glands. In these cases, the peptides are produced in the glands of the predator with sequences mimicking the prey TKs. Lastly, the TK-signaling system appears to have duplicated in Panarthropoda (comprising arthropods, onychophores, and tardigrades) to give rise to a novel type of peptides, natalisins, with a distinct receptor. The distribution and functions of natalisins are distinct from the TKs. In general, it appears that TKs are widely distributed and act in circuits at short range as neuromodulators or cotransmitters.

Truncation of neurokinin-1 receptor-Negative regulation of substance P signaling

Substance P (SP) is a tachykinin peptide, which triggers intracellular signaling in the nervous and immune systems, as well as, other local and systemic events. The interaction between SP and its receptor, neurokinin-1 receptor (NK1R), results in major downstream cellular actions, which include changes in calcium fluxes, ERK, and p21-activated kinase phosphorylation and NFκB activation. Two naturally occurring variants of the NK1R, the full-length, 407 aa receptor (NK1R-F) and the truncated, 311 aa isoform (NK1R-T), mediate the actions of SP. Receptor truncation partially disrupts signaling motifs of the carboxyl tail, a critical site for mediating NK1R signaling, resulting in a "less-efficient" receptor. Although NK1R-F is the predominant isoform in the central and peripheral nervous systems, NK1R-T is expressed in several tissues and cells, which include monocytes, NK cells, and T-cells. The SP binding domain is not affected by truncation and this site is identical in both NK1R receptor isoforms. However, while cells expressing NK1R-F respond to nanomolar concentrations of SP, monocyte and macrophage activation, mediated through NK1R-T, requires micromolar concentrations of SP in order to elicit signaling responses. Elevated plasma levels of SP are associated with increased inflammatory responses and NK1R antagonists reduce inflammation and cytokine production in vivo. This mini review presents and discusses the novel hypothesis that the expression of NK1R-T on immune system cells prevents immune activation in a milieu, which usually contains low concentrations of SP and, thus, maintains immune homeostasis. In contrast, in the activated neuronal microenvironment, when SP levels reach the threshold at tissue sites, SP promotes immune activation and modulates monocyte/macrophage polarization.

Pituitary Action of E2 in Prepubertal Grass Carp: Receptor Specificity and Signal Transduction for Luteinizing Hormone and Follicle-Stimulating Hormone Regulation

17β-estradiol (E2) is an important sex steroid produced by ovary and brain. In mammals, E2 plays an important role in hypothalamus-pituitary-gonad axis to regulate puberty onset, however, little is known about the functional role of E2 in teleost pituitary. Using prepubertal grass carp as model, three nuclear estrogen receptors (nERs: estrogen receptor alpha, estrogen receptor beta 1, and estrogen receptor beta 2) and two G protein-coupled estrogen receptors (GPER1: GPER1a and GPER1b) were isolated from grass carp pituitary. Tissue distribution analysis indicated that both nERs and GPERs were highly detected in grass carp pituitary, which suggested that E2 should play an important role in grass carp pituitary. Using primary cultured grass carp pituitary cells as model, high-throughput RNA-seq was used to examine the E2-induced differentially expressed genes (DEGs). Transcriptomic analysis showed that E2 could significantly upregulate the expression of 28 genes in grass carp pituitary cells, which were characterized into different functions including reproduction, gonad development, and central nervous system development. Further studies confirmed that E2 could induce luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion and mRNA expression in prepubertal grass carp pituitary in vivo and in vitro. In the pituitary, LH and FSH regulation by E2 were mediated by both ERβ and GPER1. Apparently, E2-induced LHβ and FSHβ mRNA expression were mediated by adenylyl cyclase/cAMP/protein kinase A, phospholipase C/inositol 1,4,5-triphosphate/protein kinase C, and Ca²⁺/calmodulin/CaM-dependent protein kinase II pathways. In addition to LH and FSH, E2 could also induce growth regulation by estrogen in breast cancer 1 (a novel regulator for pituitary development) mRNA expression in grass carp pituitary cells. These results, as a whole, suggested that E2 could play an important role in gonadotropin hormone release and pituitary development in prepubertal grass carp.

Grass Carp Follisatin: Molecular Cloning, Functional Characterization, Dopamine D1 Regulation at Pituitary Level, and Implication in Growth Hormone Regulation

Activin is involved in pituitary hormone regulation and its pituitary actions can be nullified by local production of its binding protein follistatin. In our recent study with grass carp, local release of growth hormone (GH) was shown to induce activin expression at pituitary level, which in turn could exert an intrapituitary feedback to inhibit GH synthesis and secretion. To further examine the activin/follistatin system in the carp pituitary, grass carp follistatin was cloned and confirmed to be single-copy gene widely expressed at tissue level. At the pituitary level, follistatin signals could be located in carp somatotrophs, gonadotrophs, and lactotrophs. Functional expression also revealed that carp follistatin was effective in neutralizing activin's action in stimulating target promoter with activin-responsive elements. In grass carp pituitary cells, follistatin co-treatment was found to revert activin inhibition on GH mRNA expression. Meanwhile, follistatin mRNA levels could be up-regulated by local production of activin but the opposite was true for dopaminergic activation with dopamine (DA) or its agonist apomorphine. Since GH stimulation by DA via pituitary D1 receptor is well-documented in fish models, the receptor specificity for follistatin regulation by DA was also investigated. Using a pharmacological approach, the inhibitory effect of DA on follistatin gene expression was confirmed to be mediated by pituitary D1 but not D2 receptor. Furthermore, activation of D1 receptor by the D1-specific agonist SKF77434 was also effective in blocking follistatin mRNA expression induced by activin and GH treatment both in carp pituitary cells as well as in carp somatotrophs enriched by density gradient centrifugation. These results, as a whole, suggest that activin can interact with dopaminergic input from the hypothalamus to regulate follistatin expression in carp pituitary, which may contribute to GH regulation by activin/follistatin system via autocrine/paracrine mechanisms.