Gonadotropin-Inhibitory Hormone, the Piscine Ortholog of LPXRFa, Participates in 17β-Estradiol Feedback in Female Goldfish Reproduction

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Integrated lncRNA and mRNA Transcriptome Analyses in the Ovary of Cynoglossus semilaevis Reveal Genes and Pathways Potentially Involved in Reproduction

Long non-coding RNAs (lncRNAs) have been reported to be involved in multiple biological processes. However, the roles of lncRNAs in the reproduction of half-smooth tongue sole (Cynoglossus semilaevis) are unclear, especially in the molecular regulatory mechanism driving ovarian development and ovulation. Thus, to explore the mRNA and lncRNA mechanisms regulating reproduction, we collected tongue sole ovaries in three stages for RNA sequencing. In stage IV vs. V, we identified 312 differentially expressed (DE) mRNAs and 58 DE lncRNAs. In stage V vs. VI, we identified 1,059 DE mRNAs and 187 DE lncRNAs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed that DE mRNAs were enriched in ECM-receptor interaction, oocyte meiosis and steroid hormone biosynthesis pathways. Furthermore, we carried out gene set enrichment analysis (GSEA) to identify potential reproduction related-pathways additionally, such as fatty metabolism and retinol metabolism. Based on enrichment analysis, DE mRNAs with a potential role in reproduction were selected and classified into six categories, including signal transduction, cell growth and death, immune response, metabolism, transport and catabolism, and cell junction. The interactions of DE lncRNAs and mRNAs were predicted according to antisense, cis-, and trans-regulatory mechanisms. We constructed a competing endogenous RNA (ceRNA) network. Several lncRNAs were predicted to regulate genes related to reproduction including cyp17a1, cyp19a1, mmp14, pgr, and hsd17b1. The functional enrichment analysis of these target genes of lncRNAs revealed that they were involved in several signaling pathways, such as the TGF-beta, Wnt signaling, and MAPK signaling pathways and reproduction related-pathways such as the progesterone-mediated oocyte maturation, oocyte meiosis, and GnRH signaling pathway. RT-qPCR analysis showed that two lncRNAs (XR_522278.2 and XR_522171.2) were mainly expressed in the ovary. Dual-fluorescence in situ hybridization experiments showed that both XR_522278.2 and XR_522171.2 colocalized with their target genes cyp17a1 and cyp19a1, respectively, in the follicular cell layer. The results further demonstrated that lncRNAs might be involved in the biological processes by modulating gene expression. Taken together, this study provides lncRNA profiles in the ovary of tongue sole and further insight into the role of lncRNA involvement in regulating reproduction in tongue sole.

The reproductive regulation of LPXRFa and its receptor in the hypothalamo-pituitary-gonadal axis of the spotted scat (Scatophagus argus)

Gonadotropin-inhibitory hormone (GnIH) plays a critical role in regulating gonadotropin-releasing hormone (GnRH), gonadotropin hormone (GtH), and steroidogenesis. The Lpxrfa (the piscine ortholog of GnIH) system has been found to regulate fish reproduction. To gain insight into the role of Lpxrfa in the regulation of spotted scat (Scatophagus argus) reproduction, spotted scat Lpxrfa (ssLpxrfa), and its receptor (ssLpxrfa-r) were cloned and analyzed. Tissue distribution and expression patterns at the hypothalamo-pituitary-gonadal axis (HPG axis) of sslpxrfa and sslpxrfa-r mRNA were also investigated during gonadal development of spotted scat. The open reading frame (ORF) of the sslpxrfa was 606 bp encoding 201 amino acids and includes a putative signal peptide and two mature ssLpxrfa peptides with LPXRFamide motif at their C-terminus. The sslpxrfa-r ORF was 1449 bp encoding 482 amino acids and contracted a seven-hydrophobic transmembrane (TM) domain structure. The tissue distribution showe d that the sslpxrfa was highly expressed in hypothalami, gill, and the gonads. In addition, sslpxrfa-r was highly expressed in hypothalami, pituitaries, and the gonads. Quantitative real-time polymerase chain reaction (qPCR) revealed that sslpxrfa had the highest expression in the hypothalami and pituitaries, and the lowest expression in the gonads in stage V. During gonadal development, the expression of sslpxrfa-r was gradually increased in the hypothalami but reduced in the gonads. However, no obvious trend was observed in the pituitaries. The expression of sslpxrfa and sslpxrfa-r decreased significantly after injection with 17β-estradiol (E₂). However, the expression of both sslpxrfa and sslpxrfa-r was not changed after injection with 17α-methyltestosterone(17α-MT) in the hypothalami. In addition, no changes were observed in the expression of fshβ and lhβ in the pituitaries after injecting ssLpxrfa-1. However, ssLpxrfa-2 could downregulate the expression of sbgnrh and fshβ in the hypothalami and pituitaries, respectively. Taken together, these findings suggested that ssLpxrfa may participate in E₂ feedback in reproduction and regulate the reproductive axis of spotted scat.

Arg-Vasotocin Directly Activates Isotocin Receptors and Induces COX2 Expression in Ovoviviparous Guppies

Oxytocin (OT) is a crucial regulator of reproductive behaviors, including parturition in mammals. Arg-vasopressin (AVP) is a nonapeptide homologous to Arg-vasotocin (AVT) in teleosts that has comparable affinity for the OT receptor. In the present study, ovoviviparous guppies (Poecilia reticulata) were used to study the effect of AVT on delivery mediated by the activation of prostaglandin (PG) biosynthesis via isotocin (IT) receptors (ITRs). One copy each of it and avt and two copies of itrs were identified in guppies. The results of the affinity assay showed that various concentrations of AVT and IT (10^-6, 10^-7, and 10^-8 mol/L) significantly activated itr1 (P < 0.05). In vitro experiments revealed significant upregulation (P < 0.05) of cyclooxygenase 2 (cox2), which is the rate-limiting enzyme involved in PG biosynthesis, and itr1 by AVT and IT. Furthermore, dual in situ hybridization detected positive signals for itr1 and cox2 at the same site, implying that ITR1 may regulate cox2 gene expression. Measurement of prostaglandin F_2a (PGF_2a) concentrations showed that AVT induced PGF_2a synthesis (P < 0.05) and that the effect of IT was not significant. Finally, intraperitoneal administration of PGF_2a significantly induced premature parturition of guppies. This study is the first to identify and characterize AVT and ITRs in guppies. The findings suggest that AVT promotes PG biosynthesis via ITR and that PGF_2a induces delivery behavior in ovoviviparous guppies.

FOXO1A promotes neuropeptide FF transcription subsequently regulating the expression of feeding-related genes in spotted sea bass (Lateolabrax maculatus)

FOXOs belong to the forkhead transcription factor superfamily, several of which are suggested to be involved in the control of food intake. Previously, we proved that the neuropeptide FF (NPFF) peptide was involved in feeding regulation in spotted sea bass. In the present study, seven members of the foxo family were identified in the whole genome of spotted sea bass. The distributions of these genes in different tissues were analyzed by qRT-PCR. Variations in the foxo1a and npff expression profiles during short-term starvation showed similar expression patterns. The colocalization of foxo1a and npff in the telencephalon, hypothalamus, stomach and intestine further provided evidence that foxo1a may act directly to promote the transcription of npff. Thirteen predicted FOXO1 binding sites were found in the 5' upstream region of npff. Luciferase assay results showed that FOXO1A was able to activate npff transcriptional responses by directly binding DNA response elements, and the key regulatory areas and sites of FOXO1A on the npff promoter were confirmed by deletion and site-directed mutagenesis analyses. These findings may help to elucidate the role of FOXO1 in the regulation of feeding processes in teleosts.

An estradiol-17β/miRNA-26a/cyp19a1a regulatory feedback loop in the protogynous hermaphroditic fish, Epinephelus coioides

Cyp19a1a is a key gene responsible for the production of estradiol-17β (E₂), the main functional estrogen and a major downstream regulator of reproduction in teleost fish. It is widely known that CYP19 gene expression, aromatase activity, and E₂ production can influence gonadal differentiation and sex reversal in teleost fish, but the feedback mechanisms whereby E₂ regulates cyp19a1a remain poorly understood, especially regarding the potential roles of endogenous small RNA molecules (miRNAs). Here, we identified miR-26a-5p as a regulatory factor of its predicted target gene (cyp19a1a). In vitro and in vivo studies showed that miR-26a-5p can decrease cyp19a1a expression. Furthermore, high doses of E₂ act as a repressor of miR-26a-5p. This study proposes a regulatory feedback loop whereby E₂ regulates cyp19a1a through miR-26a-5p, and suggests that this positive feedback is an important aspect of the control of E₂ production.

Examination of methods for manipulating serum 17β-Estradiol (E2) levels by analysis of blood E2 concentration in medaka (Oryzias latipes)

It is widely known that reproduction in vertebrates is regulated by the hypothalamus-pituitary-gonadal (HPG) axis. Although the mechanism of the HPG axis has been well documented in mammals, it cannot be always applied to that in non-mammalian species, which is a great disadvantage in understanding reproduction of vertebrates in general. Recently, transgenic and genome editing tools have rapidly been developed in small teleosts, and thus these species are expected to be useful for the understanding of general mechanism of reproduction in vertebrates. One of the major sex steroid hormones in female vertebrates 17β-Estradiol (E2) plays crucial roles in the formation of sexual dimorphism and the HPG axis regulation. In spite of the importance of E2 in reproductive regulation, only a few studies have analyzed blood E2 levels in small teleosts that are easily amenable to genetic manipulation. In the present study, we analyzed blood E2 concentration in medaka and demonstrated that female medaka show diurnal changes in blood E2 concentration. We then examined the best method for manipulating the circulating E2. First, we found that ovariectomy (OVX) drastically removes endogenous E2 in a day in female medaka. We examined different methods for E2 administration and revealed that feeding administration of E2-containing food is the most convenient and physiological method for mimicking the diurnal E2 changes of female medaka. On the other hand, the medaka exposed to E2 containing water showed high blood E2 concentrations, which exceeds those of environmental water, suggesting that E2 may cause bioconcentration.

NKB/NK3 system negatively regulates the reproductive axis in sexually immature goldfish (Carassius auratus)

To ascertain the significance of the Neurokinin B/Tachykinin 3 receptor (NKB/NK3) system in goldfish reproduction, two cDNAs encoding tachykinin 3 receptors, namely tacr3a and tacr3b, were cloned. Subsequent studies revealed that the downstream signalling of both Tac3rs can be activated by different NKB peptides, suggesting that the cloned receptors are biologically functional in goldfish. RT-PCR analysis showed that tacr3s are widely expressed in brain regions. During the gonadal development, tacr3a and tacr3b exhibited different expression patterns in the hypothalamus and pituitary. The actions of NKB peptides on reproductive axis was further investigated in vivo. Intraperitoneal injections of NKB peptides significantly reduced the expression of kiss2 and gonadotropin releasing hormone 3 (gnrh3) in the hypothalamus, and the expression of luteinizing hormone beta subunit (lhb) and follicle stimulating hormone beta subunit (fshb) in the pituitary in sexually immature goldfish. Taken together, our findings revealed that NKB/NK3 system plays a negative role in the reproductive axis of immature goldfish.

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.

Molecular characterization of kiss2 and differential regulation of reproduction-related genes by sex steroids in the hypothalamus of half-smooth tongue sole (Cynoglossus semilaevis)

Kisspeptin (Kiss) plays a critical role in mediating gonadal steroid feedback to the gonadotropin-releasing hormone (GnRH) neurons in mammals. However, little information regarding the regulation of kisspeptin gene by sex steroids is available in teleosts. In this study, we examined the direct actions of estradiol (E2) and testosterone (T) on hypothalamic expression of kisspeptin and other key factors involved in reproductive function of half-smooth tongue sole. As a first step, a partial-length cDNA of kiss2 was identified from the brain of tongue sole and kiss2 transcript levels were shown to be widely expressed in various tissues, notably in the ovary. Then, the actions of sex steroids on kiss2 and other reproduction-related genes were evaluated using a primary hypothalamus culture system. Our results showed that neither kiss2 nor its receptor kiss2r mRNA levels were significantly altered by sex steroids. Moreover, sex steroids did not modify hypothalamic expression of gonadotropin-inhibitory hormone (gnih) and its receptor gnihr mRNAs, either. However, E2 markedly stimulated both gnrh2 and gnrh3 mRNAs levels. Overall, this study provides insights into the role of sex steroids in the reproductive function of Pleuronectiform teleosts.