The Roles of Kisspeptin System in the Reproductive Physiology of Fish With Special Reference to Chub Mackerel Studies as Main Axis

Intraperitoneal administration of kisspeptin-10 modulates follicle maturation, gonadal steroids, calcium and metabolites in Sterlet sturgeon, Acipenser ruthenus

Kisspeptin is a multifunctional neurohormone, primarily involved in the regulation of reproduction. We tested whether peripheral administration of kisspeptin10 (KP-10) via intraperitoneal injection or slow release affects reproductive hormones and metabolites in Sterlet sturgeon (Acipenser ruthenus). Plasma and mucus 17β-estradiol (E2), and testosterone (T), plasma and follicular vitellogenin (VTG) and calcium (Ca) as well as glucose and lipids were determined. Mature Sterlet sturgeon were grouped into six groups: saline i.p injection (control), human kisspeptin (hKP-10) i.p injection; acipenser kisspeptin (aKP-10) i.p injection; hKP-10 (slow release); aKP-10 (slow-release) and no treatment control. No effect for KP-10 on sturgeon body weight was found after 4 weeks of treatment. Multivariate analysis revealed a significant disparity in plasma E2 levels. It was significantly different between groups (time, P = 0.0022). E2 in epithelia mucosa showed significant difference between and within groups in the acute group (time, P = 0.0252; treatment, P = 0.0423; time × treatment, P = 0.0429). T levels were unaffected by treatments (P > 0.05). The presence of synthetic aKP-10 led to an elevation in oocyte and plasma VTG levels (P < 0.05). Prolonged exposure to this peptide resulted in an increase in plasma calcium levels. Simultaneously, there was an augmentation in the number of mature follicles. Regardless of the duration of exposure, aKP-10 significantly elevated plasma glucose levels in Sterlet (P < 0.0). Additionally, KP-10 led to an increase in plasma lipids and cholesterol in Sterlet. Overall, our data support an involvement for KP-10 in the regulation of gonadal steroid hormones, oocyte maturation and metabolite levels in sturgeon, suggesting a positive role for this peptide in the reproductive physiology of this species.

Characterization of Kiss/Kissr system and expression profiling through developmental stages indicate kiss1 to be the active isotype in Clarias magur

Kisspeptin (Kiss) and kisspeptin receptor (Kissr) system is a key regulator of GnRH expression in several vertebrates. The Indian catfish, Clarias magur, is popular in the Indian sub-continent, and a neo-type of the Asian catfish, C. batrachus. Catfish breeding is constrained as males do not release milt captivity with/without stimulation. Magur Kiss/Kissr system comprising of kiss1, kiss2, kissr1, and kissr2 genes was characterized for the first time. Full-length mRNA was sequenced using RACE PCR. Neighbor-joining tree of predicted proteins shows one clade of teleost orthologs. Magur whole genome (NCBI GenBank) has single copies of each gene, though yet unannotated/misannotated. Anomalies in the nomenclature of earlier sequences in GenBank were noted. Relative gene expression was profiled during various ontogenic stages, in six tissues including brain and gonads at maturity, and also in brains and gonads of premature and spent fish. Expression of gnrh1, gnrhr1, and gnrhr2 was estimated concomitantly. The kiss1 was the first to be twofold upregulated (P < 0.05) at 12 h post fertilization. Kiss/Kissr genes expressed primarily in the brain, ovary, and testis. Though kiss2 was 10 times higher than kiss1, only kiss1 showed significant modulation across stages and appears to be the active isotype that regulates GnRH in magur.

Higher intake of β-glucan impairs reproduction in a female teleost, Tor putitora (Hamilton, 1822)

Although the immuno-modulatory and stress-relieving properties of β-glucan is well elucidated in humans and other animal models, including fish, its role as a dietary supplement on reproduction is extremely scarce. Therefore, in this study, adult female fish were fed one of four test diets having 0 (control), 0.5, 1, and 1.5% β-D-glucan for 130 days and its effect on reproductive performance, ovarian and liver histology, sex hormones, and transcript abundance of selected reproduction-related genes was assessed. Low dietary intake of β-glucan improved fertilization and hatching rates (p<0.05). The relative fecundity and percentage of spawning females were higher (non-significant) in 0.5% β-glucan-fed groups. Surprisingly, even after 130 days, spawning did not occur in 1.5% β-glucan-fed individuals. Irrespective of β-glucan intake, all the brooders recorded similar plasma 17β-estradiol and maturation-inducing hormone (p>0.05). Higher intake of β-glucan (1.5%) upregulated aromatase genes without a parallel increase in 17β-estradiol. However, plasma vitellogenin increased with increasing β-glucan up to 1.0% then declined at 1.5% (p<0.05). The fish that received control, 0.5, and 1.5% β-glucan recorded similar vitellogenin levels in their plasma. Significantly higher plasma cortisol was evidenced in 1.5% β-glucan fed brooders (p<0.05). Histologically, higher follicular atresia and leaking of yolk material was evidenced in 1.5% β-glucan-fed group. Liver histology revealed the highest nutrient/lipid accumulation in fish that received 1.0% and 1.5% β-glucan. This study demonstrated the stimulatory effect of β-glucan intake at a lower dose (0.5%) on reproduction. However, higher intake (1.5%) could perturb normal reproductive function in a fish model and caused an increased number of atretic follicles leading to spawning/reproductive failure.

Kisspeptin modulation of nonapeptide and cytochrome P450 aromatase mRNA expression in the brain and ovary of the catfish Heteropneustes fossilis: in vivo and in vitro studies

In Heteropneustes fossilis, kisspeptins (Kiss) and nonapeptides (NPs; vasotocin, Vt; isotocin, Itb; Val8-isotocin, Ita) stimulate the hypothalamus-pituitary-gonadal (HPG) axis, and estrogen feedback modulates the expression of these systems. In this study, functional interactions among these regulatory systems were demonstrated in the brain and ovary at the mRNA expression level. Human KISS1 (hKISS1) and H. fossilis Kiss2 (HfKiss2) produced biphasic effects on brain and ovarian vt, itb and ita expression at 24 h post injection: low and median doses produced inhibition, no change or mild stimulation, and the highest dose consistently stimulated the mRNA levels. The Kiss peptides produced an upregulation of NP mRNA expression at 24 h incubation of brain and ovarian slices by increasing the concentration of hKISS1 and HfKiss2. The kiss peptides stimulated brain cyp19a1b and ovary cyp19a1a expression, both in vivo and in vitro. Peptide234, a Kiss1 receptor antagonist, inhibited basal mRNA expression of the NPs, cyp19a1b and cyp19a1a, which was prevented by the Kiss peptides, both in vivo and in vitro. In all the experiments, HfKiss2 was more effective than hKISS1 in modulating mRNA expression. The results suggest that the NP and E2 systems are functional targets of Kiss peptides and interact with each other.

Effects of Kisspeptin on the reproductive function in the Dabry's sturgeon (Acipenser dabrynus)

Kisspeptin, a kind of neuropeptide, is involved in various physiological processes such as tumor metastasis inhibition and reproductive regulation due to its ability to interact with Kisspeptin receptor-Kissr. In teleost, Kisspeptin/Kissr system stimulates the hypothalamus-pituitary-gonadal axis (HPG axis), which is crucial for the reproductive regulation. Compared to one Kisspeptin protein Kiss1 was existed in mammals, two Kisspeptin were identified in sturgeon species, including Kiss1 and Kiss2, with specific receptors of Kissr1 and Kissr2, respectively. However, few reports described the effects of the two isoforms of Kisspeptin on the reproductive regulation in sturgeon. The core peptides of Kiss1 and Kiss2 (Kiss1-10 and Kiss2-10) of Dabry's sturgeon were successfully synthesized to explore the functional influence of Kisspeptin on the sturgeon HPG axis in the present study. The present findings suggested that intraperitoneal injection of Kiss1-10 and Kiss2-10 could significantly up-regulate the mRNA expression of Gnrh、Fsh and Lh in the hypothalamus and pituitary and the content of Lh protein in the serum. Assays of Kisspeptin-treated cells demonstrated that Kiss1-10 and Kiss2-10 can significantly promote the expression of Gnrh in hypothalamus cells and Lh and Fsh in pituitary cells of Dabry's sturgeon, indicating their direct-acting effect on pituitary cells and regulatory function on the reproductive development of sturgeon. This study described the reproductive function of the Kisspeptin in the Dabry's sturgeon for the first time, and provided supportive reference for the development of high-efficiency ripening technologies of artificially breeding sturgeon.

Long-day stimulation increases thyroid-stimulating hormone expression and affects gonadal development in chub mackerel

For seasonal breeders, photoperiodic changes are important signals that mark the start of the breeding season. Thyroid-stimulating hormone (TSH) is a glycoprotein hormone that not only promotes the secretion of thyroid hormone but also plays a key role in regulating seasonal reproduction in birds and mammals. However, whether TSH activation has been implicated as a seasonal indicator in fish breeding has not been fully investigated. In this study, we isolated tshb as a starting point to elucidate the effect of photoperiodic changes on the activation of the reproductive axis of chub mackerel. The isolated tshb was classified as tshba, which is widely conserved in vertebrates. The quantitative PCR results showed that tshb was strongly expressed in the pituitary. When female and male chub mackerel with immature gonads were reared for six weeks under different photoperiodic conditions, the gonads developed substantially in the long-day (LD) reared fish compared to those in the short-day reared fish. Real-time PCR results showed that the expression level of tshb in the pituitary gland was significantly elevated in the LD group. Although there was no difference in the gonadotropin-releasing hormone 1 gene expression level in the preoptic area of the brain, follicle-stimulating hormone and luteinizing hormone gene expression levels in the pituitary were also significantly elevated in the LD group. In conclusion, TSH is a potential mediator of seasonal information in the reproductive endocrine axis and may induce gonadal development during the breeding season of chub mackerel.

Functional evaluation of a novel kisspeptin analogue on the reproduction of female goldfish

Kisspeptin (kp) is a key regulator of reproduction, which stimulates sexual maturation and gametogenesis in mammals, amphibians, and teleosts. In the present study, to enhance the biological activity of kp10, a novel analog (referred to as M-kp10) was designed based on the endogenous goldfish variant, in which phenylalanine 6 was substituted by tryptophan and the N-terminus was acetylated. Compared with the native kp-10 and salmon gonadotropin-releasing hormone (GnRH3), the effect of M-kp10 on sexual hormones and reproductive indices as well as the expression of kiss1, cyp19a1, and kiss1ra genes in goldfish (Carassius auratus) was investigated. In practice, peptides were synthesized based on the standard Fmoc-solid-phase peptide synthesis and purified by employing RP-HPLC, followed by approving their structure using ESI-MS. The results showed that M-kp10 increased significantly 17,20β-DHP, LH, FSH and E2 as well as fecundity, hatching and fertilization percentages than the other peptides. Histological studies revealed that M-kp10 led to the faster growth of ovarian follicles compared to the kp-10 and GnRH3. The genes of cyp19a1, kiss1ra, and kiss1 were remarkably more expressed after treatment with M-kp10. In conclusion, the results indicated the superiority of M-kp10 over kp-10 in inducing sexual maturation and accelerating the percentage of fecundity, suggesting that M-kp10 could be a promising candidate for application in the artificial breeding of fish.

Linking mitochondrial dynamics and fertility: promoting fertility by phoenixin through modulation of ovarian expression of GnRH receptor and mitochondrial dynamics proteins DRP-1 and Mfn-2

Obesity is linked to reproductive disorders. Novel neuropeptide phoenixin demonstrated many therapeutic actions. In this study, we aim to evaluate phoenixin’s potential effect in obesity-induced infertility through modulating mitochondrial dynamics. Ninety adult female rats were divided to 4 groups: (I), fed with normal pellet diet; (II), given phoenixin; (III), fed with high-fat diet. Rats that developed obesity and infertility were divided to 2 groups: (III-A), received no further treatment; (III-B), given phoenixin. Our results showed that phoenixin treatment in obese infertile rats significantly decreased serum levels of insulin and testosterone and ovarian levels of dynamin-related protein1(Drp1),reactive oxygen species ROS, TNF-α, MDA, and caspase-3. Phoenixin treatment also significantly increased serum estrogen progesterone, LH, and FSH together with ovarian levels of GnRH receptor (GnRHR), mitofusin2(Mfn2), mitochondrial transmembrane potential (ΔΨm), and electron transport chain (ETC) complex-I significantly when compared with obese group. Ovarian histopathological changes were similarly improved by phoenixin. Our data demonstrate phoenixin’s role in improving obesity-induced infertility.

Sexually dimorphic distribution of kiss1 and kiss2 in the brain of yellowtail clownfish, Amphiprion clarkii

Kisspeptin system was shown to be a key factor in mediating social stress and reproduction. Yellowtail clownfish, Amphiprion clarkii, is a hermaphrodite fish, whose sex determination and gonadal development are affected by the social status of individuals. The yellowtail clownfish is a fantastic animal model to explore sex determination, but the social status and precise distribution of kiss mRNAs in the brain of this species are unknown. Hererin, a novel in situ hybridization technique, RNAscope, was used to investigate the distribution of kiss1 and kiss2 expressions in the brain of yellowtail clownfish. The coronal planes of brain showed that the kiss1 signal was mainly present in dorsal habenular nucleus (NHd) and kiss2 mRNA was widely expressed in telencephalon, midbrain, and hypothalamus, especially in dorsal part of the nucleus of the lateral recess (NRLd). Additionally, kiss1 and kiss2 signals have sexually dimorphic distribution. The kiss1 mRNA was distributed in NHd, the telencephalon, and lateral part of the diffuse nucleus of the inferior lobe (NDLIl) of females but in NHd and NDLIl of males. kiss2 signals were stronger in females than that in males. The distribution of kiss1 and kiss2 neurons in NHd of habenula and NRLd of hypothalamus may suggest that kiss genes associate environmental signaling and reproductive function in yellowtail clownfish.

Transcriptomic Profiling Revealed Signaling Pathways Associated with the Spawning of Female Zebrafish under Cold Stress

As one of the critical abiotic factors, temperature controls fish development and reproduction. However, the effects of low temperature on the transcriptional regulation of zebrafish reproduction remain largely unclear. In this study, the fecundity of zebrafish was examined after exposure to cold temperatures at 19.5 °C, 19 °C, 18.5 °C, or 18 °C. The temperature at 19 °C showed no significant influence on the fecundity of zebrafish, but temperature at 18.5 °C or 18 °C significantly blocked the spawning of females, suggesting the existence of a low temperature critical point for the spawning of zebrafish females. Based on these observations, the brains of anesthetized fish under cold stress at different cold temperatures were collected for high-throughput RNA-seq assays. Key genes, hub pathways and important biological processes responding to cold temperatures during the spawning of zebrafish were identified through bioinformatic analysis. The number of down-regulated and up-regulated genes during the temperature reduction from egg-spawning temperatures at 19.5 °C and 19 °C to non-spawning temperatures at 18.5 °C and 18 °C were 2588 and 2527 (fold change ≥ 1.5 and p-value ≤ 0.01), respectively. Venn analysis was performed to identify up- and down-regulated key genes. KEGG enrichment analysis indicated that the hub pathways overrepresented among down-regulated key genes included the GnRH signaling pathway, vascular smooth muscle contraction, C-type lectin receptor signaling pathway, phosphatidylinositol signaling system and insulin signaling pathway. GO enrichment analysis of down-regulated key genes revealed the most important biological processes inhibited under non-spawning temperatures at 18.5 °C and 18 °C were photoreceptor cell outer segment organization, circadian regulation of gene expression and photoreceptor cell maintenance. Furthermore, 99 hormone-related genes were found in the brain tissues of non-spawning and spawning groups, and GnRH signaling pathway and insulin signaling pathway were enriched from down-regulated genes related to hormones at 18.5 °C and 18 °C. Thus, these findings uncovered crucial hormone-related genes and signaling pathways controlling the spawning of female zebrafish under cold stress.

Expression and localization of neuronal nitric oxide synthase in the brain and sensory tissues of the African cichlid fish Astatotilapia burtoni

Nitric oxide (NO) produced by the enzyme neuronal nitric oxide synthase serves as an important neurotransmitter in the central nervous system that is involved in reproductive regulation, learning, sensory processing, and other forms of neural plasticity. Here, we map the distribution of nnos-expressing cells in the brain and retina of the cichlid fish Astatotilapia burtoni using in situ hybridization. In the brain, nnos-expressing cells are found from the olfactory bulbs to the hindbrain, including within specific nuclei involved in decision-making, sensory processing, neuroendocrine regulation, and the expression of social behaviors. In the retina, nnos-expressing cells are found in the inner nuclear layer, presumably in amacrine cells. We also used quantitative PCR to test for differences in nnos expression within the eye and olfactory bulbs of males and females of different reproductive states and social statuses. In the eye, males express more nnos than females, and socially dominant males express more nnos than subordinate males, but expression did not differ among female reproductive states. In the olfactory bulbs, dominant males had greater nnos expression than subordinate males. These results suggest a status-specific function for NO signaling in the visual and olfactory systems that may be important for sensory perception related to mating or territorial interactions to maintain the social hierarchy. The widespread distribution of nnos-expressing cells throughout the cichlid brain is similar to that in other teleosts, with some conserved localization patterns across vertebrates, suggesting diverse functions for this important neurotransmitter system.

Anomalous Temperature Interdicts the Reproductive Activity in Fish: Neuroendocrine Mechanisms of Reproductive Function in Response to Water Temperature

Fish are poikilotherm and small changes in water temperature can greatly affect physiological processes including reproduction, which is regulated by complex neuroendocrine mechanisms that respond to climatic events. This review provides evidence that anomalous high and low temperature may directly affect reproduction in fish by suppressing the expression of genes in the reproductive neuroendocrine system. The grass puffer, Takifugu alboplumbeus, is an excellent animal model for studying the thermal regulation of reproduction, for they exhibit periodic spawning activities, which are synchronized with seasonal, lunar and daily cycles. In the grass puffer, the expression of the genes encoding gonadotropin-releasing hormone (GnRH) 1, kisspeptin, gonadotropin-inhibitory hormone (GnIH) and their receptors were markedly suppressed in the diencephalon of fish exposed to high temperature (28°C) when compared to normal temperature (21°C), followed by the decrease in the pituitary mRNA levels for follicle-stimulating hormone (FSH), luteinizing hormone (LH) and growth hormone (GH). On the other hand, the exposure to low temperature (14°C) also inhibited the expression of gnrh1, kiss2, gnih and their receptor genes in the brain and fshb, lhb, gh and prl in the pituitary. Taken together, it is plausible that anomalous high and low temperature may be a proximate driver of termination of reproduction by suppressing the activity of the reproductive GnRH/kisspeptin/GnIH system, possibly through direct action of temperature signals at transcription level.

Cloning and expression of kiss genes and regulation of feeding in Siberian sturgeon (Acipenser baerii)

In 1996, kiss was reported to regulate feeding in mammals, but studies are limited in fish. Our study aimed to explore the possible role of kiss in the regulation of feeding in Siberian sturgeon (Acipenser baerii). kiss1 and kiss2 were cloned, and the expression patterns were analyzed in Siberian sturgeon. The complete coding regions of kiss1 and kiss2 genes were 393 and 471 bp. Both kiss1 and kiss2 showed the highest expression level in the hypothalamus. During the periprandial and fasting experiments, the expression of kiss1 and kiss2 highly significantly increased in the hypothalamus after feeding (P < 0.01). Compared with the feeding group, in hypothalamus, kiss1 expression in the fasting group highly significantly decreased (P < 0.01). In contrast, kiss2 expression had no significant difference on days 1 and 7 (P > 0.05) but highly significantly increased on day 14 (P < 0.01). Subsequently, the feeding function was verified by intraperitoneal (i.p.) injection of Kp1(10) and Kp1(10) into fish. The results showed that i.p. injection of 1 µg/g BW Kp1(10) or 0.01 µg/g BW Kp2(10) could significantly reduce 0-1 h food intake (P < 0.05) and affected the expression levels of apelin, ghrelin, leptin, nmu, etc. in the hypothalamus. These results suggested that kiss1 plays an anorexic role in both short- and long-term feeding regulation, while kiss2 plays a short-term anorexic and long-term orexigenic role. This study described kiss as a novel regulator of appetite in fish and laid the groundwork for further studies focused on physiological function. HIGHLIGHTS: • The kiss1 and kiss2 of Siberian sturgeon were cloned. • The expression levels of kiss1 and kiss2 mRNA were the highest in the hypothalamus. • Postprandial hypothalamic kiss1 and kiss2 expression levels increased in the periprandial experiment. • In the fasting test, the expression of hypothalamic kiss1 decreased after fasting, while the expression of kiss2 increased after fasting on the 14th day. • Siberian sturgeon food intake was reduced, and appetite factors expression levels in the hypothalamus were altered after intraperitoneal injection of Kp1(10) and Kp2(10).

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.

The Role of Kisspeptin in the Control of the Hypothalamic-Pituitary-Gonadal Axis and Reproduction

The discovery of kisspeptin as a critical central regulatory factor of GnRH release has given people a novel understanding of the neuroendocrine regulation in human reproduction. Kisspeptin activates the signaling pathway by binding to its receptor kisspeptin receptor (KISS1R) to promote GnRH secretion, thereby regulating the hypothalamic-pituitary-gonadal axis (HPG) axis. Recent studies have shown that kisspeptin neurons located in arcuate nucleus (ARC) co-express neurokinin B (NKB) and dynorphin (Dyn). Such neurons are called KNDy neurons. KNDy neurons participate in the positive and negative feedback of estrogen to GnRH secretion. In addition, kisspeptin is a key factor in the initiation of puberty, and also regulates the processes of female follicle development, oocyte maturation, and ovulation through the HPG axis. In male reproduction, kisspeptin also plays an important role, getting involved in the regulation of Leydig cells, spermatogenesis, sperm functions and reproductive behaviors. Mutations in the KISS1 gene or disorders of the kisspeptin/KISS1R system may lead to clinical symptoms such as idiopathic hypogonadotropic hypogonadism (iHH), central precocious puberty (CPP) and female infertility. Understanding the influence of kisspeptin on the reproductive axis and related mechanisms will help the future application of kisspeptin in disease diagnosis and treatment. In this review, we critically appraise the role of kisspeptin in the HPG axis, including its signaling pathways, negative and positive feedback mechanisms, and its control on female and male reproduction.

Hypothalamic kisspeptin and kisspeptin receptors: Species variation in reproduction and reproductive behaviours

Kisspeptin, encoded by the KISS1 gene, was first discovered as a potential metastasis suppressor gene. The prepro-kisspeptin precursor is cleaved into shorter mature bioactive peptides of varying sizes that bind to the G protein-coupled receptor GPR54 (=KISS1R). Over the last two decades, multiple types of Kiss and KissR genes have been discovered in mammalian and non-mammalian vertebrate species, but they are remarkably absent in birds. Kiss neuronal populations are distributed mainly in the hypothalamus. The KissRs are widely distributed in the brain, including the hypothalamic and non-hypothalamic regions, such as the hippocampus, amygdala, and habenula. The role of KISS1-KISS1R in humans and Kiss1-Kiss1R in rodents is associated with puberty, gonadal maturation, and the reproductive axis. However, recent gene deletion studies in zebrafish and medaka have provided controversial results, suggesting that the reproductive role of kiss is dispensable. This review highlights the evolutionary history, localisation, and significance of Kiss-KissR in reproduction and reproductive behaviours in mammalian and non-mammalian vertebrates.

Overview and New Insights Into the Diversity, Evolution, Role, and Regulation of Kisspeptins and Their Receptors in Teleost Fish

In the last two decades, kisspeptin (Kiss) has been identified as an important player in the regulation of reproduction and other physiological functions in vertebrates, including several fish species. To date, two ligands (Kiss1, Kiss2) and three kisspeptin receptors (Kissr1, Kissr2, Kissr3) have been identified in teleosts, likely due to whole-genome duplication and loss of genes that occurred early in teleost evolution. Recent results in zebrafish and medaka mutants have challenged the notion that the kisspeptin system is essential for reproduction in fish, in marked contrast to the situation in mammals. In this context, this review focuses on the role of kisspeptins at three levels of the reproductive, brain-pituitary-gonadal (BPG) axis in fish. In addition, this review compiled information on factors controlling the Kiss/Kissr system, such as photoperiod, temperature, nutritional status, sex steroids, neuropeptides, and others. In this article, we summarize the available information on the molecular diversity and evolution, tissue expression and neuroanatomical distribution, functional significance, signaling pathways, and gene regulation of Kiss and Kissr in teleost fishes. Of particular note are recent advances in understanding flatfish kisspeptin systems, which require further study to reveal their structural and functional diversity.

New Evidence for the Existence of Two Kiss/Kissr Systems in a Flatfish Species, the Turbot (Scophthalmus maximus), and Stimulatory Effects on Gonadotropin Gene Expression

Seasonal reproduction is generally controlled by the hypothalamus-pituitary-gonadal (HPG) axis in fish. Previous studies have demonstrated that the kisspeptin (Kiss)/kisspeptin receptor (Kissr) system, a positive regulator of the HPG axis, mediates the responses to environmental cues. Turbot (Scophthalmus maximus), a representative species of Pleuronectiformes, is one of the most commercially important fish species cultured in Europe and North China. However, the mechanisms by which the Kiss/Kissr system regulates the reproductive axis of turbot according to seasonal changes, especially photoperiod, have not been clearly characterized. In the current study, the cDNA sequences of kiss2/kissr2, along with kiss1/kissr3 which was thought to be lost in flatfish species, were cloned and functionally characterized. The kiss1, kiss2, and kissr3 transcripts were highly detected in the brain and gonad, while kissr2 mRNA was only abundantly expressed in the brain. Moreover, kiss/kissr mRNAs were further examined in various brain areas of both sexes. The kiss1, kissr2, kissr3 mRNAs were highly expressed in the mesencephalon, while a substantial degree of kiss2 transcripts were observed in the hypothalamus. During annual reproductive cycle, both kiss and kissr transcript levels declined significantly from the immature to mature stages and increased at the degeneration stage in the brains of both sexes, especially in the mesencephalon and hypothalamus. The ovarian kiss1, kiss2, and kissr2 mRNA levels were highest at the vitellogenic stage (mature stage), while expression of kissr3 was highest at the immature stage. The testicular kiss and kissr transcripts were highest in the immature and degeneration stages, and lowest at the mature stage. In addition, intraperitoneal injection of Kiss1-10 and Kiss2-10 significantly stimulated mRNA levels of pituitary lhβ, fhsβ, and gthα. In summary, two Kiss/Kissr systems were firstly proven in a flatfish species of turbot, and it has a positive involvement in controlling the reproduction of the Kiss/Kissr system in turbot. The results will provide preliminary information regarding how the Kiss/Kissr system controls seasonal reproduction in turbot broodstock.

New evidence for SPX2 in regulating the brain-pituitary reproductive axis of half-smooth tongue sole (Cynoglossus semilaevis)

Spexin (SPX) is an evolutionarily conserved neuropeptide, which was first identified in human proteome by data mining. Two orthologs (SPX1 and SPX2) are present in some non-mammalian species, including teleosts. It has been demonstrated that SPX1 is involved in reproduction and food intake, whereas the functional role of SPX2 is still absent in any vertebrate. The aim of the current study was to evaluate the actions of intraperitoneal injection of endogenous SPX2 peptide on the expression levels of some key reproductive genes of the brain-pituitary axis in half-smooth tongue sole. Our data showed an inhibitory action of SPX2 on brain gnih, spx1, tac3 and pituitary gthα, lhβ mRNA levels. However, SPX2 had no significant effect on brain gnihr, gnrh2, gnrh3, kiss2, kiss2r, spx2 expression or pituitary gh expression. On the other hand, SPX2 induced an increase in pituitary fshβ expression. Taken together, our results provide initial evidence for the involvement of SPX2 in the regulation of reproduction in vertebrates, which is in accordance with previous studies on SPX1.

Kisspeptin as autocrine/paracrine regulator of human ovarian cell functions: Possible interrelationships with FSH and its receptor

The present study aims to examine the role of kisspeptin (KP), FSH, and its receptor (FSHR), and their interrelationships in the control of basic human ovarian granulosa cells functions. We investigated: (1) the ability of granulosa cells to produce KP and FSHR, (2) the role of KP in the control of ovarian functions, and (3) the ability of KP to affect FSHR and to modify the FSH action on ovarian functions. The effects of KP alone (0, 10 and 100 ng/mL); or of KP (10 and 100 ng/mL) in combination with FSH (10 ng/mL) on cultured human granulosa cells were assessed. Viability, markers of proliferation (PCNA and cyclin B1) and apoptosis (bax and caspase 3), as well as accumulation of KP, FSHR, and steroid hormones, IGF-I, oxytocin (OT), and prostaglandin E2 (PGE2) release were analyzed by the Trypan blue exclusion test, quantitative immunocytochemistry, and ELISA. KP given at a low dose (10 ng/mL) stimulated viability, proliferation, inhibited apoptosis, promoted the release of progesterone (P4), estradiol (E2), IGF-I, OT, and PGE2, the accumulation of FSHR, but not testosterone (T) release. KP given at a high dose (100 ng/mL) had the opposite, inhibitory effect. FSH stimulated cell viability, proliferation and inhibited apoptosis, promoted P4, T, E2, IGF-I, and OT, but not PGE2 release. Furthermore, KP at a low dose promoted the stimulatory effect of FSH on viability, proliferation, P4, E2, and OT release, promoted its inhibitory action on apoptosis, but did not modify its action on T, IGF-I, and PGE2 output. KP at a high dose prevented and inverted FSH action. These results suggest an intra-ovarian production and a functional interrelationship between KP and FSH/FSHR in direct regulation of basic ovarian cell functions (viability, proliferation, apoptosis, and hormones release). The capability of KP to stimulate FSHR, the ability of FSH to promote ovarian functions, as well as the similarity of KP (10 ng/mL) and FSH action on granulosa cells' viability, proliferation, apoptosis, steroid hormones, IGF-I, OT, and PGE2 release, suggest that FSH influence these cells could be mediated by KP. Moreover, the capability of KP (100 ng/mL) to decrease FSHR accumulation, basal and FSH-induced ovarian parameters, suggest that KP can suppress some ovarian granulosa cell functions via down-regulation of FSHR. These observations propose the existence of the FSH-KP axis up-regulating human ovarian cell functions.

Kisspeptin Receptor on the Sperm Surface Reflects Epididymal Maturation in the Dog

Alongside the well-known central modulatory role, the Kisspeptin system, comprising Kiss1, its cleavage products (Kisspeptins), and Kisspeptin receptor (Kiss1R), was found to regulate gonadal functions in vertebrates; however, its functional role in the male gamete and its localization during maturation have been poorly understood. The present study analyzed Kisspeptin system in dog testis and spermatozoa recovered from different segments of the epididymis, with focus on Kiss1R on sperm surface alongside the maturation during epididymal transit, demonstrated by modification in sperm kinetic, morphology, and protamination. The proteins Kiss1 and Kiss1R were detected in dog testis. The receptor Kiss1R only was detected in total protein extracts from epididymis spermatozoa, whereas dot blot revealed Kiss1 immunoreactivity in the epidydimal fluid. An increase of the Kiss1R protein on sperm surface along the length of the epididymis, with spermatozoa in the tail showing plasma membrane integrity and Kiss1R protein (p < 0.05 vs. epididymis head and body) was observed by flow cytometry and further confirmed by epifluorescence microscopy and Western blot carried on sperm membrane preparations. In parallel, during the transit in the epididymis spermatozoa significantly modified their ability to move and the pattern of motility; a progressive increase in protaminization also occurred. In conclusion, Kisspeptin system was detected in dog testis and spermatozoa. Kiss1R trafficking toward plasma membrane along the length of the epididymis and Kiss1 in epididymal fluid suggested a new functional role of the Kisspeptin system in sperm maturation and storage.

Seasonal expression and distribution of kisspeptin1 (kiss1) in the ovary and testis of freshwater catfish, Clarias batrachus: A putative role in steroidogenesis

The central role of kisspeptin (kiss) in mammalian reproduction is well established; however, its intra-gonadal role is poorly addressed. Moreover, studies investigating intra-gonadal role of kiss in fish reproduction are scanty, contradictory and inconclusive. The expression of kiss1 mRNA has been detected in the fish brain, and functionally attributed to the regulation of reproduction, feeding and behavior. The kiss1 mRNA has also been demonstrated in tissues other than the brain in some studies, but its cellular distribution and role at the tissue level have not been adequately addressed in fish. Therefore, an attempt was made in the present study to localize kiss1 in gonadal cells of the freshwater catfish, Clarias batrachus. This study reports the presence of kiss1 in the theca cells and granulosa cells of the ovarian oocytes and interstitial cells in the testis of the catfish. The role of kiss1 in the ovary and testis of the catfish was also investigated using kiss1 receptor (kiss1r) antagonist (p234). The p234 treatment decreased the production of 17β-estradiol in ovary and testosterone in the testis by lowering the activities of 3β-hydroxysteroid dehydrogenase and 17β-hydroxysteroid dehydrogenase under both, in vivo as well as in vitro conditions. The p234 treatment also arrested the progression of oogenesis, as evident from the low number of advancing/advanced oocytes in the treated ovary in comparison to the control ovary. It also reduced the area and perimeter of the seminiferous tubules in the treated catfish testis. Thus, our findings suggest that kiss is involved in the regulation of gonadal steroidogenesis, independent of known endocrine/ autocrine/ paracine regulators, and thereby it accelerates gametogenic processes in the freshwater catfish.

Hormonal and neuroendocrine control of reproductive function in teleost fish

Reproduction is one of the important physiological events for the maintenance of the species. Hormonal and neuroendocrine regulation of teleost requires multiple and complex interactions along the hypothalamic-pituitary-gonad (HPG) axis. Within this axis, gonadotropin-releasing hormone (GnRH) regulates the synthesis and release of gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Steroidogenesis drives reproduction function in which the development and differentiation of gonads. In recent years, new neuropeptides have become the focus of reproductive physiology research as they are involved in the different regulatory mechanisms of these species' growth, metabolism, and reproduction. However, especially in fish, the role of these neuropeptides in the control of reproductive function is not well studied. The study of hormonal and neuroendocrine events that regulate reproduction is crucial for the development and success of aquaculture.

Effects of LPXRFamide peptides on chub mackerel gonadotropin secretion

Gonadotropin-inhibitory hormone (GnIH), a neuropeptide, suppresses gonadotropin (GTH) secretion in birds and mammals. In fish, the GnIH homolog LPXRFamide (LPXRFa) produces mature peptides with species-dependent effects on sexual reproduction. Here, we investigated the effects of LPXRFa on GTH secretion in the chub mackerel (cm; Scomber japonicus). We cloned cmlpxrfa (603 bp) and cmlpxrfa-r (1,416 bp). Additionally, we isolated lpxrfa from the bluefin tuna (Thunnus orientalis) to confirm the conservation of the LPXRFa mature sequence. Phylogenetic analysis showed that the LPXRFa precursor protein produces three mature peptides, LPXRFa-1, -2, and - 3, in both species. Reverse transcription-quantitative PCR revealed that cmlpxrfa is expressed in the hypothalamus and thalamus and midbrain (T.MB), and sexual differences were observed. Receptor expression was observed in the pre-optic area, hypothalamus, T.MB, and pituitary. Female hypothalamic lpxrfa expression did not change during puberty. Reporter gene assay showed that LPXRFa induced receptor activation via the CRE and SRE signaling pathways. However, in the presence of forskolin, an intracellular cyclic AMP enhancer, none of the LPXRFa could suppress receptor activity. The in vitro bioassay results showed that gonadotropin-releasing hormone-1 (GnRH1) had no effect on follicle-stimulating hormone (FSH) secretion, whereas the three LPXRFa significantly increased FSH secretion in pituitary cells from male chub mackerel. Contrarily, GnRH1 and three LPXRFa significantly increased luteinizing hormone (LH) secretion. The in vivo administration of LPXRFa had no effect on fshb and lhb expression in pre-pubertal and mature male chub mackerel. Overall, cmLPXRFa lacks the ability to suppress GTH secretion but can promote GTH secretion.

Gametogenic and steroidogenic action of kisspeptin-10 in the Asian catfish, Clarias batrachus: Putative underlying mechanistic cascade

Unlike mammals, two kisspeptins genes encoding, kiss1 and kiss2 are detected in fishes with highly varied and contradictory difference in their reproductive activities. The present study was undertaken to examine the direct action of kisspeptin-10 and its role in gonadal activities in the gonadally quiescent Asian catfish using native mammalian kisspeptin decapeptide (KP-10) involving in vivo and in vitro approaches. The in vivo KP-10 treatment caused precocious onset of gametogenesis and its rapid progression, as was evident from the appearance of advanced stages of ovarian follicles in ovary, and advanced germ cells (spermatocytes/ spermatids) in the testis of the treated Clarias batrachus in comparison to the control gonads. It also elevated the steroid levels in gonads of the catfish in vivo and in vitro conditions. Simultaneously, it increased the expressions of key steroidogenic enzymes like 3β-HSD, 17β-HSD, and StAR protein, responsible for transfer of cholesterol from outer to inner membrane of the mitochondria of steroidogenic cells. Concurrently, it augmented the activities of 3β-HSD and 17β-HSD in the ovarian explants. The expressions of MAPK component (pERK1/2 and ERK1/2) were also up-regulated by KP-10 in gonadal explants. Thus, the data suggest that kisspeptin-10 stimulates gametogenesis by enhancing gonadal steroid production. The study also describes the putative mechanistic cascade of steroidogenic actions of kisspeptin-10 in the catfish so much so in teleost fish. The study also suggests that, kisspeptin may act locally to regulate gonadal activities in an autocrine/paracine manner, independent of known extra-gonadal factors in the catfish.

The effect of different exogenous kisspeptins on sex hormones and reproductive indices of the goldfish (Carassius auratus) broodstock

Despite several studies on fish hormone therapy, finding new candidates may provide more reproductive efficiency in artificial propagation. Kisspeptins, being upstream of the hypothalamic-pituitary-gonadal axis, appear to play a key role in the reproduction process. In the present study, the effect of different variants of kisspeptide, including goldfish (Carassius auratus) kiss1 kisspeptin (Kiss1), human kisspeptin (Hkiss), and their combination (Kiss1 + H), on the reproductive indices of goldfish broodstock in comparison to Ovaprim (a typical synthetic Gnrh hormone) was investigated. Peptides (Kiss1 and Hkiss) were synthesized using a solid-phase synthesis approach. Kiss1 and Hkiss were injected at a dose of 100 μg kg-1 body weight, blood samples were taken 6 h after injection and sex hormones (E2, Dhp, and 11-Kt), gonadotropins (Lh and Fsh), cortisol and reproductive indices (fecundity, fertilization and hatching percentage) were measured. The results showed a significant increase of plasma sex hormones and gonadotropins in fish treated with kisspeptins. In addition, the cortisol and lipoprotein lipase in Kiss1, Hkiss and Kiss1 + H were remarkably increased compared to Ovaprim. In conclusion, kisspeptins could be a more suitable candidate than Ovaprim for accelerating and synchronizing oocyte maturation in the fisheries industry.

Roles of melatonin in the teleost ovary: A review of the current status

Melatonin, the neurohormone mainly synthesized in and secreted from the pineal gland of vertebrates following a circadian rhythm, is an important factor regulating various physiological processes, including reproduction. Recent data indicate that melatonin is also synthesized in the ovary and that it acts directly at the level of the ovary to modulate ovarian physiology. In some teleosts, melatonin is reported to affect ovarian steroidogenesis. The direct action of melatonin on the ovary could be a possible factor promoting oocyte maturation in teleosts. A role for melatonin in follicle rupture during ovulation in the teleost medaka has recently emerged. In addition, melatonin is suggested to affect oocyte maturation by its antioxidant activity. However, the molecular mechanisms underlying these direct effects of melatonin are largely unknown.

Expression dynamics of the genes for the hypothalamo-pituitary-gonadal axis in tiger puffer (Takifugu rubripes) at different reproductive stages

Tiger puffer, Takifugu rubripes, a commercially important long-distance migratory fish, return to specific spawning grounds for reproduction. To clarify reproductive neuroendocrine system of the tiger puffer, the changes in the expression levels of the genes encoding three gonadotropin-releasing hormones (GnRHs), gonadotropin-inhibitory hormone (GnIH), GnIH receptor (GnIH-R), kisspeptin and kisspeptin receptor in the brain and gonadotropin (GTH) subunits, growth hormone (GH) and prolactin (PRL) in the pituitary were examined in the tiger puffer captured in the wild at different reproductive stages, namely immature and mature fish of both sexes, and post-ovulatory females that were obtained by hormonal treatment. The amounts of three gnrh mRNAs, gnih, gnih-r, fshb and lhb were substantially increased in the mature fish compared to the immature fish, especially in the females, and these augmented expressions were drastically decreased in the post-ovulatory females. gh expression showed a slight increase in the mature males. In contrast, kiss2, kiss2r and prl did not show significant changes in the males but significantly decreased in the post-ovulatory females. The present results demonstrate the expression dynamics of the hypothalamo-pituitary-gonadal axis genes associated with the reproductive conditions and the possible involvement of the GnRH/GnIH/GTH system in the regulation of the sexual maturation and spawning in the wild tiger puffer.

Kisspeptins, new local modulators of male reproduction: A comparative overview

Spermatogenesis is a complex process that leads to the production of male gametes within the testis through the coordination of mitotic, meiotic and differentiation events, under a deep control of endocrine, paracrine and autocrine modulators along the Hypothalamus-pituitary-gonad (HPG) axis. The kisspeptin system plays a fundamental role along the HPG axis as it is the main positive modulator upstream of the hypothalamic neurons that secrete the Gonadotropin Releasing Hormone (GnRH), the decapeptide that supports pituitary gonadotropins and the production of gonadal sex steroid. Currently, kisspeptins and their receptor, KISS1R, have a recognized activity in the central control of puberty onset, sex maturation, reproduction and sex-steroid feedback mechanisms in both animal models and human. However, kisspeptin signaling has been widely reported in peripheral tissues, particularly in the testis of mammalian and non-mammalian vertebrates, with functions related to Leydig cells physiology and steroid biosynthesis, spermatogenesis progression and spermatozoa functions, but its mandatory role within the testis is still a matter of discussion. This review provides a summary of the main intratesticular effects of kisspeptin in vertebrates, via a comparative approach. Particular emphasis was devoted to data from the anuran amphibian Pelophylax esculentus, the first animal model in which the direct intratesticular activity of kisspeptin was reported.

Expression profile of kisspeptin2 and gonadotropin-releasing hormone2 mRNA during photo-thermal and melatonin treatments in the female air-breathing catfish Heteropneustes fossilis

In seasonally breeding vertebrates, extrinsic factors like photoperiod and temperature are major determinants, controlling the annual reproductive cycle. In teleosts, kisspeptin, which occurs in two molecular forms: kisspeptin1 (Kiss1) and kisspetin2 (Kiss2), has been reported to alter gonadotropin (Lh and Fsh) secretion but its effect on gonadotropin-releasing hormone (Gnrh) secretion is not unequivocally proved. In the catfish Heteropneustes fossilis, we isolated and characterized kiss2 and gnrh2 cDNAs and the present work reports effects of altered photo-thermal conditions and melatonin (MT, a pineal hormone) on their expressions in the brain. The exposure of the catfish to long photoperiod (LP, 16 h light) at normal temperature (NT) or high temperature (HT, 28 °C) at normal photoperiod (NP) for 14 or 28 days stimulated both kiss2 and gnrh2 expression in both gonad resting and preparatory phases with the combination of LP + HT eliciting maximal effects. Short photoperiod (SP, 8 h light) under NT or HT altered the gene expression according to the reproductive phase and temperature. MT that mediates photo-thermal signals to the brain inhibited brain kiss2 and gnrh2 gene expression in the NP + HT, LP + NT, and SP + NT groups. The altered photo-thermal conditions elicited changes in steroidogenic pathway as evident from changes in plasma E₂, progesterone, and testosterone levels. The results show that brain kiss2-gnrh2 signaling is involved in photo-thermal-mediated mechanisms controlling reproduction.

Nutrient regulation of somatic growth in teleost fish. The interaction between somatic growth, feeding and metabolism

This review covers the current knowledge on the regulation of the somatic growth axis and its interaction with metabolism and feeding regulation. The main endocrine and neuroendocrine factors regulating both the growth axis and feeding behavior will be briefly summarized. Recently discovered neuropeptides and peptide hormones will be mentioned in relation to feeding control as well as growth hormone regulation. In addition, the influence of nutrient and nutrient sensing mechanisms on growth axis will be highlighted. We expect that in this process gaps of knowledge will be exposed, stimulating future research in those areas.

Kisspeptin Influences the Reproductive Axis and Circulating Levels of microRNAs in Senegalese Sole

Kisspeptin regulates puberty and reproduction onset, acting upstream of the brain–pituitary–gonad (HPG) axis. This study aimed to test a kisspeptin-based hormonal therapy on cultured Senegalese sole (G1) breeders, known to have reproductive dysfunctions. A single intramuscular injection of KISS2-10 decapeptide (250 µg/kg) was tested in females and males during the reproductive season, and gonad maturation, sperm motility, plasma levels of gonadotropins (Fsh and Lh) and sex steroids (11-ketotestosterone, testosterone and estradiol), as well as changes in small non-coding RNAs (sncRNAs) in plasma, were investigated. Fsh, Lh, and testosterone levels increased after kisspeptin injection in both sexes, while sperm analysis did not show differences between groups. Let7e, miR-199a-3p and miR-100-5p were differentially expressed in females, while miR-1-3p miRNA was up-regulated in kisspeptin-treated males. In silico prediction of mRNAs targeted by miRNAs revealed that kisspeptin treatment might affect paracellular transporters, regulate structural and functional polarity of cells, neural networks and intracellular trafficking in Senegalese sole females; also, DNA methylation and sphingolipid metabolism might be altered in kisspeptin-treated males. Results demonstrated that kisspeptin stimulated gonadotropin and testosterone secretion in both sexes and induced an unanticipated alteration of plasma miRNAs, opening new research venues to understand how this neuropeptide impacts in fish HPG axis.

Kisspeptin and GnRH interactions in the reproductive brain of teleosts

It is well known that gonadotropin-releasing hormone (Gnrh) has a key role in reproduction by regulating the synthesis and release of gonadotropins from the anterior pituitary gland of all vertebrates. About 25 years ago, another neuropeptide, kisspeptin (Kiss1) was discovered as a metastasis suppressor of melanoma cell lines and then found to be essential for mammalian reproduction as a stimulator of hypothalamic Gnrh and regulator of puberty onset. Soon after, a kisspeptin receptor (kissr) was found in the teleost brain. Nowadays, it is known that in most teleosts the kisspeptin system is composed of two ligands, kiss1 and kiss2, and two receptors, kiss2r and kiss3r. Even though both kisspeptin peptides, Kiss1 and Kiss2, have been demonstrated to stimulate gonadotropin synthesis and secretion in different fish species, their actions appear not to be mediated by Gnrh neurons as in mammalian models. In zebrafish and medaka, at least, hypophysiotropic Gnrh neurons do not express Kiss receptors. Furthermore, kisspeptinergic nerve terminals reach luteinizing hormone cells in some fish species, suggesting a direct pituitary action. Recent studies in zebrafish and medaka with targeted mutations of kiss and/or kissr genes reproduce relatively normally. In zebrafish, single gnrh mutants and additionally those having the triple gnrh3 plus 2 kiss mutations can reproduce reasonably well. In these fish, other neuropeptides known to affect gonadotropin secretion were up regulated, suggesting that they may be involved in compensatory responses to maintain reproductive processes. In this context, the present review explores and presents different possibilities of interactions between Kiss, Gnrh and other neuropeptides known to affect reproduction in teleost fish. Our intention is to stimulate a broad discussion on the relative roles of kisspeptin and Gnrh in the control of teleost reproduction.

Kiss2 but not kiss1 is involved in the regulation of social stress on the gonad development in yellowtail clownfish, Amphiprion clarkii

The yellowtail clownfish (Amphiprion clarkii) is a hermaphrodite fish, whose sex differentiation and gonad development are closely related to its social status. The kisspeptin/KissR system is regarded as a key factor mediating social stress on reproductive regulation. In order to understand the effects of social rank stress on the yellowtail clownfish gonadal differentiation, full-length cDNAs of two paralogous genes encoding kisspeptin (kiss1 and kiss2) and KissR (kissr2 and kissr3) were cloned and characterized. The results of real-time PCR showed that kiss1 was primarily expressed in the hypothalamus, and kiss2/kissr2 were abundantly expressed in the liver, while kissr3 was almost exclusively concentrated in the cerebellum and pituitary. Moreover, both Kiss1-10 and Kiss2-10 peptides could initiate downstream signaling pathways by interacting with cognate receptors expressed in eukaryotic cells. Among the three social status groups, the mRNA levels of kiss2 in the hypothalamus and pituitary as well as kissr2 in the pituitary were significantly higher in subordinate individuals (nonbreeders) than dominate individuals (females and males); while the mRNA levels of kissr3 in the hypothalamus and gonad were low in subordinate individuals. Furthermore, the plasma estradiol (E₂) and testosterone (T) levels were higher in subordinate than dominate individuals. This study shows that kiss2 is involved in the regulation of social stress on the gonad development in the yellowtail clownfish, but not kiss1.

Transcriptome characterization of BPG axis and expression profiles of ovarian steroidogenesis-related genes in the Japanese sardine

Background

The clupeoid fishes are ecologically and commercially important fish species worldwide that exhibit a high level of population fluctuation, accompanied by alteration of reproductive traits. However, knowledge about their reproductive physiology in order to understand mechanisms underlying such population dynamics is limited. The endocrine system along with the brain–pituitary–gonadal (BPG) axis is critical for regulating reproduction. The aims of this study were to provide transcript data and genes related to the BPG axis, and to characterize the expression profiles of ovarian steroidogenesis-related genes in the Japanese sardine (Sardinops melanostictus, Clupeidae).

Results

RNA sequencing was performed using the sardine brain, pituitary, and gonad in both sexes. A total of 290,119 contigs were obtained and 115,173 non-redundant ORFs were annotated. The genes differentially expressed between ovary and testis were strongly associated with GO terms related to gamete production. The tissue-specific profile of the abundance of transcripts was characterized for the major regulators in the BPG axis, such as gonadotropin-releasing hormone, gonadotropin, and steroidogenic enzyme. By comparing between ovary and testis, out of eight different 17β-hydroxysteroid dehydrogenase (Hsd17b) genes identified, higher hsd17b7 expression was found in testis, whereas higher expression of hsd17b8, hsd17b10, hsd17b12a, and hsd17b12b was found in ovary. The cDNAs encoding key endocrine factors in the ovarian steroidogenic pathway were cloned, sequenced, and quantitatively assayed. In the pituitary, follicle-stimulating hormone beta peaked during vitellogenesis, while luteinizing hormone beta peaked at the completion of vitellogenesis. In the ovary, follicle-stimulating hormone receptor and luteinizing hormone receptor were upregulated from mid- to late phase of vitellogenesis. Furthermore, three steroidogenic enzyme genes (cyp11a1, cyp17a1, and cyp19a1a) gradually increased their expression during ovarian development, accompanying a rise in serum estradiol-17β, while 3β-hydroxysteroid dehydrogenase and steroidogenic acute regulatory protein did not change significantly.

Conclusions

This is the first report of deep RNA sequencing analysis of Japanese sardine, in which many key genes involved in the BPG axis were identified. Expression profiles of ovarian steroidogenesis-related genes provide a molecular basis of the physiological processes underlying ovarian development in the sardine. Our study will be a valuable resource for clarifying the molecular biology of clupeoid fishes.

Identification of kisspeptin2 cDNA in the catfish Heteropneustes fossilis: Expression profile, in situ localization and steroid modulation

Kisspeptin (Kiss) is considered an upstream regulator of gonadotropin-releasing hormone in mammals but its role in non-mammalian vertebrates is not unequivocally established. In the catfish Heteropneustes fossilis, a 605 bp long cDNA was identified from the brain by cloning as well as by retrieving from the catfish transcriptome database. The open reading frame (ORF, 93-405 bp) codes for a 113 amino acids long precursor protein. Homology and phylogenetic analyses showed that the predicted protein belongs to the vertebrate Kiss2 type with a high degree of conservation in the Kiss2-10 region (FNFNPFGLRF). The kiss2 transcripts were expressed highly in the brain and gonads in a dimorphic manner with a female bias. In the brain, kiss2 transcripts showed regional differences with higher expression in the medulla oblongata and forebrain regions. The kiss2 transcripts showed significant seasonal variations with the highest expression in the brain in spawning phase and in the gonads in prespawning phase. The kiss2 transcripts were localized in the brain (nucleus preopticus, habenular nucleus, nucleus recessus posterioris, nucleus recessus lateralis) and stratum periventriculare (radial glial cells) of optic tectum, pituitary and ovary (follicular layer and germinal vesicle). Ovariectomy (1, 2, 3 and 4 weeks) decreased brain kiss2 mRNA levels and a single injection of estradiol-17β (E₂; 0.5 μg/g body weight) in 3- week ovariectomized (OVX) and sham operated fish resulted in an increase in the transcript levels after 24 h. The E₂ receptor antagonist Tamoxifen (TMX) produced biphasic effects on the kiss2 expression in the dose- response study. TMX inhibited the expression in the OVX fish, but elicited a stimulatory effect in the OVX + E₂-treated fish. Testosterone (T) decreased, and progesterone (P₄) inhibited (resting phase) or stimulated (prespawning phase) the transcript level in 3-week OVX fish. In the 3-week sham groups, E₂ increased, and TMX, T and P₄ inhibited the kiss2 transcript levels. The results suggest that Kiss2 is an important regulator of the brain- pituitary- gonadal- endocrine axis, and in habenular and optic tectum functions.

Nesfatin-1-like peptide suppresses hypothalamo-pituitary-gonadal mRNAs, gonadal steroidogenesis, and oocyte maturation in fish†

Nucleobindin (Nucb)-1 and Nucb2 are DNA and Ca2+ binding proteins with multiple functions in vertebrates. Prohormone convertase-mediated processing of Nucb2 results in the production of biologically active nesfatin-1. Nesfatin-1 is involved in the regulation of reproduction in many vertebrates, including fish. Our lab originally reported a nesfatin-1-like peptide (Nlp) encoded in Nucb1 that exhibits nesfatin-1-like metabolic effects. We hypothesized that Nlp has a suppressive role in the reproductive physiology of fish. In this research, whether Nlp regulates reproductive hormones and oocyte maturation in fish were determined. Single intraperitoneal (IP) injection of goldfish Nlp (50 ng/g body weight) suppressed salmon and chicken gonadotropin-releasing hormone (sgnrh and cgnrh2), gonadotropin-inhibiting hormone (gnih) and its receptor (gnihr), and kisspeptin and brain aromatase mRNA expression in the hypothalamus of both male and female goldfish. In the pituitary, Nlp decreased mRNAs encoding lhb, fshb and kisspeptin and its receptor, while a significant increase in gnih and gnihr was observed. In the gonads, lh (only in male fish) and fsh receptor mRNAs were also significantly downregulated in Nlp-injected fish. Sex-specific modulation of gnih, gnihr, and kisspeptin system in the gonads was also observed. Nlp decreased sex steroidogenic enzyme encoding mRNAs and circulating levels of testosterone and estradiol. In addition, incubation of zebrafish ovarian follicles with Nlp resulted in a reduction in oocyte maturation. These results provide evidence for a robust role for Nlp in regulating reproductive hormones in goldfish and oocyte maturation in zebrafish, and these effects resemble that of nesfatin-1.

Leptin stimulates gonadotropin release and ovarian development in marine teleost chub mackerel

Leptin transmits information about energy stored in the periphery to the reproductive axis and is an essential signal for puberty initiation in mammals; however, to date, few studies have focused on the direct effects of leptin stimulation on reproductive factors in fish. This study demonstrated the effect of leptin stimulation on important reproductive factors and ovarian development in the marine teleost chub mackerel (Scomber japonicus). We prepared recombinant leptin and conducted functional analyses through in vitro bioassays using primary pituitary cells, long-term leptin treatment administered to pre-pubertal females, and intracerebroventricular (ICV) administration. The results showed that leptin stimulation strongly induced gonadotropin (follicle-stimulating hormone: FSH and luteinizing hormone: LH) secretion from pituitary cells collected from pre-pubertal females, and that long-term leptin treatment significantly promoted ovarian development and triggered pubertal onset. Furthermore, ICV administration of leptin did not affect kisspeptin gene expression but significantly upregulated gonadotropin-releasing hormone 1 (gnrh1), fshb and lhb gene expression in sexually immature females. These results strongly suggest leptin as an important signal for reproductive-axis activation in chub mackerel.

Phoenixin-20 Stimulates mRNAs Encoding Hypothalamo-Pituitary-Gonadal Hormones, is Pro-Vitellogenic, and Promotes Oocyte Maturation in Zebrafish

Phoenixin-20 (PNX-20) is a bioactive peptide with hormone-like actions in vertebrates. In mammals, PNX stimulates hypothalamo-pituitary-gonadal hormones and regulate reproductive processes. Our immunohisto/cytochemical studies show PNX-like and the putative PNX receptor, SREB3-like immunoreactivity in the gonads of zebrafish, and in zebrafish liver (ZFL) cells. Intraperitoneal injection of zebrafish PNX-20 upregulates mRNAs encoding both salmon gonadotropin-releasing hormone (GnRH), and chicken GnRH-II and kisspeptin and its receptor in zebrafish hypothalamus. Similarly, luteinizing hormone receptor mRNA expression in the testis, follicle-stimulating hormone receptor in the ovary, and the kisspeptin system were upregulated in the gonads of PNX-20 injected fish. We also observed the upregulation of genes involved in the sex steroidogenic pathway (cyp11a1, cyp17a1, 17βhsd, cyp19a1a) in the gonads of PNX-20 administered fish. PNX-20 upregulates the expression of vitellogenin isoforms and estrogen receptor (esr2a and 2b) mRNAs in ZFL cells in vitro. Meanwhile, siRNA-mediated knockdown of PNX-20 resulted in the downregulation of all vitellogenin transcripts, further suggesting its possible role in vitellogenesis. PNX-20 treatment resulted in a significant increase in germinal vesicle breakdown in zebrafish follicles in vitro. Collectively, these results provide strong evidence for PNX-20 effects on the HPG axis and liver to promote reproduction in zebrafish.

Alanine scanning and characterization of core peptides in Scombridae fish family for construction of Kiss1 super analog

Chronic Kiss1 administration strongly promotes gonadal development in immature chub mackerel (cm) (Scomber japonicus). Here, we performed an Alanine scanning (Ala-scanning) of Kiss1 to determine its key residues. Additionally, we examined functional peptides from 16 Scombridae species to develop maturation-inducing super-analogs that can be used universally in Scombridae species. In the Ala-scanning of Kiss1-15 (QDMSSYNFNSFGLRY), substitution of Gln¹ and Asp² did not affect agonistic activity. This suggests that peptides could be downsized. Furthermore, it is possible that Phe⁸ can be substituted by unnatural amino acids that are difficult to degrade. In molecular cloning, only Scomber showed a 16-residue form as a putative mature peptide. The other genera, did not have a His residue at the N-terminal, which indicated that the functional peptide was 15 residues and the second and third residues from the N-terminal showed variation between interspecies. Next, we examined the binding affinity of various synthetic Kiss1 core peptides in Scombridae interspecies using an SRE-Luc reporter system. We cloned Kiss1 receptors (KissR1) from bluefin tuna (bft) (Thunnus orientalis) and Japanese Spanish mackerel (jsm) (Scomberomorus niphonius) for the first time. In binding affinity with cmKissR1, bftKissR1, and jsmKissR1, the species specificity of the second residue from the N-terminus in each ligand could be ignored, but the difference in the third residue strongly affected receptor binding. Scombridae species possess the same Kiss1 system but the structure of the functional peptide might be species-specific.

Expression of the kisspeptin/gonadotropin-releasing hormone (GnRH) system in the brain of female Chinese sucker (Myxocyprinus asiaticus) at the onset of puberty

The kisspeptin-kisspeptin receptor (kissr)-gonadotropin-releasing hormone (GnRH) system plays a key role in regulating the onset of puberty in mammals. However, the role of this system in fish is still unclear. We examined the relative gene expression patterns for kiss1, kiss2, kissr2, sGnRH, and pjGnRH in all parts of the brains of Chinese sucker (Myxocyprinus asiaticus) females at the prepubertal and pubertal stages by using real-time PCR. We also analyzed the expression of kiss1 and GnRH1 via immunofluorescence. Two variants of kisspeptin; a variant of kissr (kissr2); and two variants of GnRH, pjGnRH (GnRH1), and sGnRH (GnRH3), were expressed in all parts of the brain. The mRNA expression of kiss1 was higher in the telencephalon, mesencephalon, and diencephalon at the pubertal stage than at the prepubertal stage, and the expression of kiss2 was higher in only the telencephalon. The expression of kissr2 was higher in all parts of the brain, except the medulla, at the pubertal stage than at the prepubertal stage. pjGnRH was highly expressed in all parts of the brain at the pubertal stage, whereas sGnRH expression showed no distinct changes, except in the epencephalon. Strong kiss1 and weak GnRH-1 immunoreactivity was observed in the pineal gland, lateral tuberal nucleus (NLT), and ventral part of the NLT in the diencephalon of the Chinese sucker females at the pubertal stage. Our results suggest that the kiss1-kissr2-pjGnRH system was expressed highly at the onset of pubertal female Chinese sucker.

Hypothalamic- and pituitary-derived growth and reproductive hormones and the control of energy balance in fish

Most endocrine systems in the body are influenced by the hypothalamic-pituitary axis. Within this axis, the hypothalamus delivers precise signals to the pituitary gland, which in turn releases hormones that directly affect target tissues including the liver, thyroid gland, adrenal glands and gonads. This action modulates the release of additional hormones from the sites of action, regulating key physiological processes, including growth, metabolism, stress and reproduction. Pituitary hormones are released by five distinct hormone-producing cell types: somatotropes (which produce growth hormone), thyrotropes (thyrotropin), corticotropes (adrenocorticotropin), lactotropes (prolactin) and gonadotropes (follicle stimulating hormone and luteinizing hormone), each modulated by specific hypothalamic signals. This careful and distinct organization of the hypothalamo-pituitary axis has been classically associated with the existence of many lineal axes (e.g., the hypothalamic-pituitary-gonadal axis) in charge of the control of the different physiological processes. While this traditional concept is valid, it is becoming apparent that hormones produced by the hypothalamo-pituitary axis have diverse effects. For instance, gonadotropin-releasing hormone II has been associated with a suppressive effect on food intake in fish. Likewise, growth hormone has been shown to influence appetite, swimming activity and aggressive behavior in fish. This review will focus on the hypothalamic and pituitary hormones classically involved in regulating growth and reproduction, and will attempt to provide a general overview of the current knowledge on their actions on energy balance and appetite in fish. It will also give a brief perspective of the role of some of these peptides in integrating feeding, metabolism, growth and reproduction.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Expression patterns of kiss2 and gpr54-2 in Monopterus albus suggest these genes may play a role in sex reversal in fish

Due to its exceptionally small genome size and protogynous hermaphroditism, Monopterus albus has been proposed as a model for vertebrate sexual development. The Kiss/GPR54 system is a central regulator of sexual development in most vertebrates, but its role in sex reversal remains hypothetical. In contrast to mammals, fishes often possess more than one copy of the kiss and gpr54 genes. Our objectives were to identify all kiss/gpr54 genes in the genome of M. albus and to assess their involvement in sex reversal via their expression patterns (qPCR) in females, males, and intersex specimens. We identified only two genes: kiss2 and gpr54‐2. kiss2 expression was extremely high in the gonads of males, intermediate in females, and low in intersex; and reduced in all tissues of intersex. gpr54 expression was also extremely high in the gonads of males, high in intersex, but low in females. gpr54 expression in brain was high in all three sexes. In conclusion, (a) kiss1 has been functionally replaced in M. albus; (b) the functions of gpr54‐2 in brain are not sex‐specific; (c) kiss2 appears to undergo a ‘reset’ in the expression during the sex change; and (d) sex‐specific expression patterns in the gonads indicate that these two genes may play a role in sex reversal in fish.

Molecular identification of GnIH and its potential role in reproductive physiology and male pregnancy of the lined seahorse (Hippocampus erectus)

The gonadotropin-inhibitory hormone (GnIH) plays a negative role in the hypothalamic-pituitary-gonadal (HPG) axis by inhibiting gonadotropin secretion in vertebrates. Male pregnancy and ovoviviparous behavior are unique phenomena among vertebrates. To better understand the neuroendocrine regulatory mechanisms in ovoviviparous fish with male pregnancy, we identified the orthologous GnIH gene in the lined seahorse (Hippocampus erectus). The full-length cDNA of the GnIH precursor was 658 base pairs with an open reading frame of 528 base pairs that encoded a 175-amino acid prepro-GnIH peptide. The seahorse GnIH precursor contained two putative LPXRFamide peptides. Both seahorse LPXRFa-1 and LPXRFa-2 were found to be unique among vertebrates. The synteny blocks of GnIH gene loci were conserved in mammals and teleosts. Tissue distribution analysis revealed that seahorse GnIH mRNA was mainly expressed in the hypothalamus, with relatively high levels observed in the brood pouch. The expression patterns of seahorse GnIH during different reproductive stages and pregnancy stages were also detected, and GnIH mRNA expression was significantly reduced during the early puberty stage. In addition, GnIH mRNA expression was significantly increased during the pregnancy stage compared to non-pregnancy stages. In summary, our results reveal the existence of GnIH in ovoviviparous fish and suggest its involvement in regulation of reproductive behavior and male pregnancy in the male seahorse.

Androgen signaling in male fishes: Examples of anti-androgenic chemicals that cause reproductive disorders

Similar to other vertebrates, androgens regulate spermatogenesis in fishes. In teleosts, the main androgen is 11-Ketotestosterone (11-KT), which is oxidized testosterone (T) at the C₁₁ position. Compared to T, 11-KT is a nonaromatizable steroid, and does not convert to 17β-estradiol. However, circulatory levels of both T and 11-KT undergo seasonal variations along with testicular development. Physiological functions of androgens are mediated via androgen receptor (Ar). So far, nuclear Ar and membrane Ar have been identified in fishes. In the present study, we reviewed androgen biosynthesis in fishes, compared molecular structure of nuclear Ar in models of mammals and fishes, and investigated the mechanisms of action of environmental contaminants that differentially disrupt androgen signaling in fish reproduction. In the latter case, the adverse effects of vinclozolin (VZ) and bis(2-ethylhexyl) phthalate (DEHP) are compared. Both VZ and DEHP are capable of decreasing sperm quality in males. Vinclozolin causes an increase in 11-KT production associated with increases in kisspeptin (kiss-1) and salmon gonadotropin-releasing hormone (gnrh3) mRNA levels as well as circulatory levels of luteinizing hormone (Lh). In contrast, DEHP inhibits 11-KT production associated with a decrease in circulatory Lh levels. However, DEHP-inhibited 11-KT production is not associated with changes in kiss-1 and gnrh3 mRNA levels. Studies also show that VZ alters ar mRNA levels, while DEHP is without effect. These suggest that VZ and DEHP act differentially to cause androgen-dependent reproductive disorder in male fishes. Molecular analyses of the nuclear AR show that both DNA and ligand binding domains (DBD and LBD, respectively) are highly conserved within models of mammals and fishes. A phylogeny tree of the AR shows distinct clusters between mammals and fishes. In fishes, subtypes of Arα and Arβ are also separated in distinct clusters. Thus, further studies need to generate ar knockout fish model to better elucidate androgen regulation of reproduction in fishes via Ar.