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.

Lunar Age-Dependent Oscillations in Expression of the Genes for Kisspeptin, GnIH, and Their Receptors in the Grass Puffer during the Spawning Season

Grass puffer is a semilunar-synchronized spawner: spawning occurs on beaches only for several days of spring tide around new moon (lunar age 0) and full moon (lunar age 15) every 2 weeks from spring to early summer. To investigate the role of kisspeptin and gonadotropin-inhibitory hormone (GnIH) in the semilunar-synchronized spawning, lunar age-dependent expression of the genes encoding kisspeptin (kiss2), kisspeptin receptor (kissr2), GnIH (gnih), GnIH receptor (gnihr), gonadotropin-releasing hormone 1 (GnRH1) (gnrh1), and three gonadotropin (GTH) subunits (gpa, fshb, lhb) was examined in the male grass puffer, which was kept in an aquarium under natural light condition in a lunar month during the spawning period. In the brain, both kiss2 and kissr2 showed lunar variations with a peak at lunar age 10, while both gnih and gnihr showed semilunar variations with two peaks at lunar age 0 and 20. On the other hand, gnrh1 showed semilunar variation with two peaks at lunar age 0 and 15. In the pituitary, kiss2, kissr2, gnih, and gnihr showed similar variations to those shown in the brain. The fshb and lhb mRNA levels showed semilunar variations with two peaks at lunar age 0 and 15. The present study shows lunar and semilunar oscillations of kiss2/kissr2 and gnih/gnihr expressions, respectively, with their peaks around spring tide in the brain and pituitary along with the semilunar expressions of gnrh1 and the pituitary GTH subunit genes. These results suggest that the lunar age-dependent expressions of the kisspeptin, GnIH, and their receptor genes may be primarily important in the control of the precisely timed semilunar spawning of the grass puffer.

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.

Cloning, Expression Analysis and SNP Screening of the kiss1 Gene in Male Schizothorax biddulphi

Schizothorax biddulphi is an endemic fish distributed only in southern Xinjiang, China. Due to overfishing, water conservancy facilities, and other factors, as well as inherent biological limitations, resource recovery is quite difficult. For endangered fish with slow growth, late sexual maturity, and insufficient natural population supplementation, large-scale artificial reproduction and breeding are important for restoring resources. Therefore, it is urgent to optimize the reproductive regulation methods of the fish. The kiss1 gene is a key regulator of the reproductive regulation cascade, and identifying and analyzing the role of kiss1 are important for further elucidating the reproductive mechanism of S. biddulphi. To understand the characteristics of the kiss1 of S. biddulphi, the full-length cDNA sequence of kiss1 was obtained in this study, and its tissue expression specificity and association with phenotypic traits were analyzed in male fish. The full-length cDNA sequence of kiss1 in S. biddulphi was 658 bp, with an ORF of 327 bp, and encoded a 108-amino acid, unstable protein. Homology results indicated that kiss1 was highly conserved. qPCR showed kiss1 expression in different tissues in male S. biddulphi, with the highest expression in the gonads, followed by muscle, and significantly lower expression in the swim bladder, pituitary gland, heart, hypothalamus, gill, fin, liver, eye, and mid-kidney. qPCR revealed three SNP loci in the exonic region of kiss1. The c.3G>T locus was significantly correlated (p < 0.05) with gonad mass and the maturation coefficient in S. biddulphi. These results will help uncover the reproductive endocrinology network of S. biddulphi, improve artificial breeding technology for fish, and unveil new directions for breeding excellent strains of S. biddulphi and molecular marker-assisted breeding.

A chronic exposure to bisphenol A reduces sperm quality in goldfish associated with increases in kiss2, gpr54, and gnrh3 mRNA and circulatory LH levels at environmentally relevant concentrations

The bisphenol A (BPA)-disrupted reproductive functions have been demonstrated in male animals. In fish, it has been shown that environmentally relevant concentrations of BPA decrease sperm quality associated with inhibition of androgen biosynthesis. However, BPA effects on neuroendocrine regulation of reproduction to affect testicular functions are largely unknown. In the present study, reproductive functions of hypothalamus and pituitary were studied in mature male goldfish exposed to nominal 0.2, 2.0 and 20.0 μg/L BPA. At 90 d of exposure, sperm volume, velocity, and density and motility were decreased in goldfish exposed to 0.2, 2.0, and 20.0 μg/L BPA, respectively (p < 0.05). At 30 d of exposure, there were no significant changes in circulatory LH levels and mRNA transcripts of kiss1, Kiss2, gpr54, and gnrh3. At 90 d of exposure, circulatory LH levels showed trends toward increases in BPA exposed goldfish, which was significant in those exposed to 2.0 μg/L (P < 0.05). At this time, Kiss2, gpr54, and gnrh3 mRNA levels were increased in goldfish exposed to any concentrations of BPA (p < 0.05). This study shows that BPA-diminished sperm quality was accompanied by an increase in circulatory LH levels associated with increases in mRNA transcripts of upstream neuroendocrine regulators of reproduction in goldfish. Further, this is the first study to report circulatory levels of LH in fish exposed to BPA.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

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.

Kisspeptin Exhibits Stimulatory Effects on Expression of the Genes for Kisspeptin Receptor, GnRH1 and GTH Subunits in a Gonadal Stage-Dependent Manner in the Grass Puffer, a Semilunar-Synchronized Spawner

Kisspeptin has an important role in the regulation of reproduction by directly stimulating the secretion of gonadotropin-releasing hormone (GnRH) in mammals. In non-mammalian vertebrates, there are multiple kisspeptins (Kiss1 and Kiss2) and kisspeptin receptor types, and the two kisspeptins in teleosts have different effects depending on fish species and reproductive stages, serving reproductive and non-reproductive functions. In the grass puffer, Takifugu alboplumbeus, which has only a single pair of kiss2 and kissr2, both genes display seasonal, diurnal, and circadian oscillations in expression in association with the periodic changes in reproductive functions. To elucidate the role of kisspeptin in this species, homologous kisspeptin peptide (gpKiss2) was administered at different reproductive stages (immature, mature and regressed) and the expression levels of the genes that constitute hypothalamo-pituitary-gonadal axis were examined in male grass puffer. gpKiss2 significantly elevated the expression levels of kissr2 and gnrh1 in the brain and kissr2, fshb and lhb in the pituitary of the immature and mature fish. No noticeable effect was observed for kiss2, gnih, gnihr, gnrh2 and gnrh3 in the brain and gpa in the pituitary. In the regressed fish, gpKiss2 was ineffective in stimulating the expression of the gnrh1 and GTH subunit genes, while it stimulated and downregulated the kissr2 expression in the brain and pituitary, respectively. The present results indicate that Kiss2 has a stimulatory role in the expression of GnRH1/GTH subunit genes by upregulating the kissr2 expression in the brain and pituitary at both immature and mature stages, but this role is mostly ineffective at regressed stage in the grass puffer.

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.

Expression of Kisspeptin 1 in the Brain of the Adult Sea Lamprey Petromyzon marinus

Kisspeptin peptides play major roles in the regulation of reproduction and puberty onset in mammals. While most mammals only have one kisspeptin gene, other jawed vertebrates present two or three genes. Recent data also revealed the presence of two genes in lampreys (jawless vertebrates). However, apart from gene sequence data, there is almost no information on the kisspeptinergic system of lampreys. Here, we report phylogenetic and cluster-based analyses showing that the duplication of the ancestral kisspeptin gene occurred before the separation of jawless and jawed vertebrates. We also studied the expression of the kisspeptin transcripts in the brain of post-metamorphic juveniles and upstream migrating adult sea lampreys. Our in situ hybridization results revealed expression of kisspeptin 1 in hypothalamic neurons, which indicates that the hypothalamic expression of kisspeptins is an ancestral character in vertebrates. We also observed the presence of kisspeptin 1 expressing neurons in the paratubercular (posterior tubercle) nucleus of the diencephalon. This is the first description of the presence of kisspeptin 1 expressing neurons in this brain region in any vertebrate. We did not detect expression of kisspeptin 2 in the juvenile or adult sea lamprey brain with in situ hybridization. Our data provides an anatomical basis to study the role of kisspeptin 1 in the hypothalamic-pituitary system of lampreys and the contribution of diencephalic kisspeptinergic neurons to different circuits of the lamprey brain.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

Characterization of kiss2/kissr2 system in largemouth bass (Micropterus salmoides) and Kiss2-10 peptide regulation of the hypothalamic-pituitary-gonadal axis

The kisspeptin system, which lies upstream of the hypothalamic-pituitary-gonadal (HPG) axis, is believed to function as a regulator of reproduction in teleosts. In this study, we isolated and characterized kiss2 and its receptor kissr2 in largemouth bass (Micropterus salmoides). The complete coding sequences of kiss2 and kissr2 were 375 and 1134 bp long and encoded precursor proteins 124 and 377 amino acid long, respectively. Real-time PCR showed that kiss2 and kissr2 were primarily expressed in the HPG axis. The expression profile of kiss2 and kissr2 varied with gonadal development, with the highest and lowest expression levels being detected during the immature and final maturation stages, respectively. Intraperitoneal injection of exogenous Kiss2-10 peptide increased the transcript levels of gnrh3, kissr2, fshβ, lhβ, ar, and er2 within 24 h (p < 0.05), as well as plasma levels of 17β-estradiol and testosterone. Histological analysis indicated that chronic administration of exogenous Kiss2-10 peptide accelerated vitellogenesis in females and spermatogenesis in males. Further, in situ hybridization revealed that kiss2 is expressed in the ooplasm and vitelline envelope of oocytes and the spermatocytes of testes. In addition, experiments using gonad tissue primary cell cultures indicated that exogenous Kiss2-10 peptide stimulates the expression of reproduction-related genes. Collectively, our findings indicate that the kiss2/kissr2 system in largemouth bass is involved in regulating gonadal development through the HPG axis.

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.

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

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

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.

In silico analysis of kiss2, expression studies and protein-protein interaction with gonadotropin-releasing hormone 2 (GnRH2) and luteinizing hormone beta (LHβ) in Heteropneustes fossilis

Kisspeptins, encoded by the kiss genes, are neuropeptides that regulate the onset of puberty, maturation of gonads, and fertility in higher vertebrates including fishes. The gene ontology suggests that kisspeptin plays an important role not only in reproduction but also in cell signaling, immune response and metabolic processes, and to decipher protein-protein interactions, computational approach has been favored. The present investigation focuses on the detailed structural analysis and molecular docking of kiss2 gene using in silico tools. A putative kiss2 protein of 113 amino acids was encoded by an open reading frame of 342 bp kiss2 gene. The protein is of 13.12 kDa with isoelectric point of 9.45. The secondary structure of the protein indicates more than 50% random coils, followed by 34% of alpha helix and 13% extended strand. The protein was found to be extracellular and secretory in nature. Since, protein-protein interactions play a very crucial role in every cellular process and due to unavailability of crystal structure of our protein of interest in fishes computational approach has been employed. The 3D PDB modeling and the molecular docking of kiss2, Gonadotropin-releasing hormone 2 (GnRH2) and luteinizing hormone beta (LHβ) proteins in fishes have been demonstrated applying protein-docking approach. Molecular interactions of kiss2 protein were the highest with kisspeptin receptor 2 and lowest for the neuropeptide FF-amide peptide precursor protein. Expression of kiss2 transcripts, mainly in the brain and ovary of H. fossilis, supports its hypothalamic-pituitary-gonadal axis signaling and reproductive function. Further, changes in expression patterns of kiss2 mRNA during different developmental stages, indicate its potential role in embryonic development also. The present study conclusively reveals interaction of kiss2 with other neuropeptides. Prediction of binding structures and identification of key residues in protein-protein interaction illustrate direct interaction among these proteins, playing a cardinal role in neuroendocrine regulation of reproduction in catfish. Communicated by Ramaswamy H. Sarma.

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.

Effects of Kiss2 on the Expression of Gonadotropin Genes in the Pituitary of Nile Tilapia (Oreochromis niloticus)

Kisspeptin, expressed mainly in the hypothalamus, stimulates gonadotropin-releasing hormone neurons to facilitate reproduction. In some model animals, the kisspeptin is also expressed in the pituitary. Recently, a pathway has been suggested in which kisspeptin acts directly on the pituitary to secretion of gonadotropin in mammals. In the present study, pituitaries of the Nile tilapia (Oreochromis niloticus) were cultured at different concentrations of kisspeptin-10 (Kp-10, FNYNPLSLRF) for 3 hours to observe the effect of kisspeptin on the expression of follicle-stimulating hormone β subunit (fshβ) gene and luteinizing hormone β subunit (lhβ) gene. Pituitary tissues were cultured with 0.1 μM of Kp-10, luteinizing hormone releasing hormone (LHRH), or LHRH+Kp-10 for 3, 6, 12, and 24 hours to investigate changes in the expression of fshβ and lhβ mRNA. Pituitaries cultured with high concentration of Kp-10 more than 0.1 μM for 3 hours exhibited a significant increase of fshβ mRNA expression, but not lhβ mRNA. The expression of both fshβ and lhβ mRNA increased after 6 hours in 0.1 μM of Kp-10 medium in comparison with that in the control medium. Tissues cultured in the LHRH medium however exhibited increased expression of both genes not only at 6 but also 12 hours. There were no significant differences of fshβ and lhβ gene expression in tissues cultured with LHRH+KP-10 medium compared with the control. These results suggested that although kisspeptin plays an important role in fshβ and lhβ expression in the pituitary of Nile tilapia, its action is far more complicated than expected.

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.

Existence and functions of a kisspeptin neuropeptide signaling system in a non-chordate deuterostome species

The kisspeptin system is a central modulator of the hypothalamic-pituitary-gonadal axis in vertebrates. Its existence outside the vertebrate lineage remains largely unknown. Here, we report the identification and characterization of the kisspeptin system in the sea cucumber Apostichopus japonicus. The gene encoding the kisspeptin precursor generates two mature neuropeptides, AjKiss1a and AjKiss1b. The receptors for these neuropeptides, AjKissR1 and AjKissR2, are strongly activated by synthetic A. japonicus and vertebrate kisspeptins, triggering a rapid intracellular mobilization of Ca²⁺, followed by receptor internalization. AjKissR1 and AjKissR2 share similar intracellular signaling pathways via G_αq/PLC/PKC/MAPK cascade, when activated by C-terminal decapeptide. The A. japonicus kisspeptin system functions in multiple tissues that are closely related to seasonal reproduction and metabolism. Overall, our findings uncover for the first time the existence and function of the kisspeptin system in a non-chordate species and provide new evidence to support the ancient origin of intracellular signaling and physiological functions that are mediated by this molecular system.

Climate change impacts on fish reproduction are mediated at multiple levels of the brain-pituitary-gonad axis

Anthropogenic emissions of carbon dioxide in the atmosphere have generated rapid variations in atmospheric composition which drives major climate changes. Climate change related effects include changes in physico-chemical proprieties of sea and freshwater, such as variations in water temperature, salinity, pH/pCO₂ and oxygen content, which can impact fish critical physiological functions including reproduction. In this context, the main aim of the present review is to discuss how climate change related effects (variation in water temperature and salinity, increases in duration and frequency of hypoxia events, water acidification) would impact reproduction by affecting the neuroendocrine axis (brain-pituitary-gonad axis). Variations in temperature and photoperiod regimes are known to strongly affect sex differentiation and the timing and phenology of spawning period in several fish species. Temperature mainly acts at the level of gonad by interfering with steroidogenesis, (notably on gonadal aromatase activity) and gametogenesis. Temperature is also directly involved in the quality of released gametes and embryos development. Changes in salinity or water acidification are especially associated with reduction of sperm quality and reproductive output. Hypoxia events are able to interact with gonad steroidogenesis by acting on the steroids precursor cholesterol availability or directly on aromatase action, with an impact on the quality of gametes and reproductive success. Climate change related effects on water parameters likely influence also the reproductive behavior of fish. Although the precise mechanisms underlying the regulation of these effects are not always understood, in this review we discuss different hypothesis and propose future research perspectives.

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.

The mRNA expression patterns of kisspeptins, GnRHs, and gonadotropins in the brain and pituitary gland of a tropical damselfish, Chrysiptera cyanea, during the reproductive cycle

The sapphire devil (Chrysiptera cyanea) is a tropical damselfish that undergoes active reproduction under long-day conditions. To elucidate the physiological regulation of the brain-pituitary-gonadal axis in female sapphire devil, we cloned and characterized the genes of two kisspeptins (kiss1 and kiss2), three gonadotropin-releasing hormones (gnrh1, gnrh2, gnrh3), and the β-subunit of two gonadotropins (fshβ and lhβ) and investigated the gene expression changes during ovarian development. Quantitative polymerase chain reaction analyses in various brain parts revealed high expression levels of kiss1, kiss2, and gnrh2 in the diencephalon; gnrh2 and gnrh3 in the telencephalon; and fshβ and lhβ in the pituitary. In situ hybridization (ISH) analyses revealed positive signals of kiss1 in the dorsal and ventral habenular nucleus and of kiss2 in the dorsal and ventral parts of the nucleus of the lateral recess. This analysis showed gnrh1 expression in the preoptic area (POA), suggesting that GnRH1 plays a stimulating role in the secretion of gonadotropins from the pituitary of the sapphire devil. High transcription levels of kiss1, kiss2, gnrh1, gnrh2, fshβ, and lhβ were observed in the brain during the late vitellogenic stage, suggesting their involvement in the physiological processes of vitellogenesis. Immersion of fish in estradiol-17β (E2)-containing seawater resulted in increased expression of kiss2 and gnrh1 in their brains. This study showed that kiss-expressing neurons in the diencephalon are influenced by E2, leading to upregulation of gnrh1 in the POA and of fshβ and lhβ in the pituitary during vitellogenesis.

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.

The Kiss2/GPR54 system stimulates the reproductive axis in male black porgy, Acanthopagrus schlegelii

Although it is well established that the Kiss1/GPR54 system stimulates the reproductive axis in mammals, its functional roles, especially in male reproduction of non-mammalian species, is less clear. In this study, we have isolated the full-length kiss2 and gpr54 cDNAs from black porgy (Acanthopagrus schlegelii). The Kiss2 precursor expressed from kiss2 comprises 124 amino acids and contains a highly conserved 10-amino acid sequence, Kiss2-10 (FNFNPFGLRF). GPR54 comprises 375 amino acid residues and contains distinct characteristics of G protein-coupled receptors. Real-time PCR analysis indicated that kiss2 and gpr54 were expressed highly in the brain regions. Moreover, intraperitoneal injection of porgy Kiss2-10 could stimulate genes expression of the gpr54, gnrh1, gnrh3, fshβ, lhβ, p450c17, star, and ar, and the serum testerone level in male black porgy. Our findings demonstrate that the Kisspeptin stimulates the male reproductive axis in black porgy.

Distribution of Kiss2 receptor in the brain and its localization in neuroendocrine cells in the zebrafish

Kisspeptin is a hypothalamic neuropeptide, which acts directly on gonadotropin-releasing hormone (GnRH)-secreting neurons via its cognate receptor (GPR54 or Kiss-R) to stimulate GnRH secretion in mammals. In non-mammalian vertebrates, there are multiple kisspeptins (Kiss1 and Kiss2) and Kiss-R types. Recent gene knockout studies have demonstrated that fish kisspeptin systems are not essential in the regulation of reproduction. Studying the detailed distribution of kisspeptin receptor in the brain and pituitary is important for understanding the multiple action sites and potential functions of the kisspeptin system. In the present study, we generated a specific antibody against zebrafish Kiss2-R (=Kiss1Ra/GPR54-1/Kiss-R2/KissR3) and examined its distribution in the brain and pituitary. Kiss2-R-immunoreactive cell bodies are widely distributed in the brain including in the dorsal telencephalon, preoptic area, hypothalamus, optic tectum, and in the hindbrain regions. Double-labeling showed that not all but a subset of preoptic GnRH3 neurons expresses Kiss2-R, while Kiss2-R is expressed in most of the olfactory GnRH3 neurons. In the posterior preoptic region, Kiss2-R immunoreactivity was seen in vasotocin cells. In the pituitary, Kiss2-R immunoreactivity was seen in corticotropes, but not in gonadotropes. The results in this study suggest that Kiss2 and Kiss2-R signaling directly serve non-reproductive functions and indirectly subserve reproductive functions in teleosts.

Effects of neuropeptides and sex steroids on the pituitary-gonadal axis of pre-pubertal F1 wreckfish (hāpuku) Polyprion oxygeneios in vivo: Evidence of inhibitory effects of androgens

The ability to advance puberty in broodstock that have a long generation interval and mature at large size is a highly valuable tool in contemporary aquaculture enterprise. Juvenile male and female wreckfish 'hāpuku' (Polyprion oxygeneios), a candidate for commercialization in aquaculture, were subjected to treatment for 8weeks with two implants, one containing steroid (blank; estradiol-17β, E2; 11-ketotestosterone, KT; 17 α-methyltestosterone, MT), the other peptide (blank; gonadotropin-releasing hormone analog, GnRHa; kisspeptin, Kiss2-12). The expression of target genes (glycoprotein homone α-subunit, gpa; follicle stimulating-hormone β-subunit, fshb; luteinizing hormone β-subunit, lhb; GnRH receptor, gnrhr) in the pituitary was assayed by quantitative PCR. KT and MT decreased mRNA levels of all target genes in both male and female hāpuku, suggestive of a strong inhibitory tone by these steroid hormones. E2, GnRHa and Kiss2-12 were largely ineffective, regardless of whether they were administered alone or in combination with steroid implants. Clear differences in release and/or clearance rates between E2 and KT from implants were evident, in part explaining our observations. Advancement of puberty was not achieved, and we pose that different hormone doses and/or administration during more advanced stages of gonadogenesis need to be considered to move this field forward.

Mapping Molecular Datasets Back to the Brain Regions They are Extracted from: Remembering the Native Countries of Hypothalamic Expatriates and Refugees

This article focuses on approaches to link transcriptomic, proteomic, and peptidomic datasets mined from brain tissue to the original locations within the brain that they are derived from using digital atlas mapping techniques. We use, as an example, the transcriptomic, proteomic and peptidomic analyses conducted in the mammalian hypothalamus. Following a brief historical overview, we highlight studies that have mined biochemical and molecular information from the hypothalamus and then lay out a strategy for how these data can be linked spatially to the mapped locations in a canonical brain atlas where the data come from, thereby allowing researchers to integrate these data with other datasets across multiple scales. A key methodology that enables atlas-based mapping of extracted datasets-laser-capture microdissection-is discussed in detail, with a view of how this technology is a bridge between systems biology and systems neuroscience.

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

Kisspeptin, a novel neuropeptide product of the Kiss1 gene, activates the G protein-coupled membrane receptor G protein-coupled receptor 54 (now termed Kiss1r). Over the last 15 years, the importance of the kisspeptin system has been the subject of much debate in the mammalian research field. At the heart of the debate is whether kisspeptin is an absolute upstream regulator of gonadotropin-releasing hormone secretion, as it has been proposed to be the master molecule in reproductive events and plays a special role not only during puberty but also in adulthood. The teleostean kisspeptin system was first documented in 2004. Although there have been a number of kisspeptin studies in various fish species, the role of kisspeptin in reproduction remains a subject of controversy and has not been widely recognized. There is an extensive literature on the physiological and endocrinological bases of gametogenesis in fish, largely derived from studying small, model fish species, and reports on non-model species are limited. The reason for this discrepancy is the technical difficulty inherent in developing rigorous experimental systems in many farmed fish species. We have already established methods for the full life-cycle breeding of a commercially important marine fish, the chub mackerel (cm), and are interested in understanding the reproductive function of kisspeptins from various perspectives. Based on a series of experiments clarifying the role of the brain-pituitary-gonad axis in modulating reproduction in cm, we theorize that the kisspeptin system plays an important role in the reproduction of this scombroid species. In this review article, we provide an overview of kisspeptin studies in cm, which substantially aids in elucidating the role of kisspeptins in fish reproduction.

Biological Significance of Kisspeptin-Kiss 1 Receptor Signaling in the Habenula of Teleost Species

Kisspeptin is a neuropeptide, encoded by kisspeptin 1 (KISS1)/Kiss1 gene, which primarily acts as the regulator of reproductive functions via its receptor, kisspeptin receptor (KissR) in vertebrates. In the brain, Kiss1 gene is mainly expressed in the hypothalamic region, but KissR gene is widely distributed throughout the brain, suggesting that kisspeptin-KissR system may be involved in not only reproductive, but also non-reproductive functions. In non-mammalian vertebrates, there are two or more kisspeptin and KissR types. The zebrafish (Danio rerio) possess two kisspeptin (Kiss1 and Kiss2) and their respective receptors [Kiss1 receptor (KissR1) and KissR2]. In the brain of zebrafish, while Kiss2 is expressed in the preoptic-hypothalamic area, Kiss1 is predominantly expressed in the habenula, an evolutionarily conserved epithalamic structure. Similarly, KissR1 is expressed only in the habenula, while KissR2 is widely distributed in the brain, suggesting that the two kisspeptin systems play specific roles in the brain. The habenular Kiss1 is involved in the modulation of the raphe nuclei and serotonin-related behaviors such as fear response in the zebrafish. This review summarizes the roles of multiple kisspeptin-KissR systems in reproductive and non-reproductive functions and neuronal mechanism, and debates the biological and evolutional significance of habenular kisspeptin-KissR systems in teleost species.

Conformational analysis of a synthetic fish kisspeptin 1 peptide in membrane mimicking environments

Kisspeptin 1 is a neuropeptide hormone of the RFamide family, which act as an upstream regulator of brain-pituitary-gonad (BPG) axis in most vertebrates including teleosts. In the present study, a 16 amino acid long putative mature bioactive peptide (kiss 1) from preprokisspeptin 1 of golden mahseer, Tor putitora (Hamilton, 1822), was synthesized and characterized using an integrated (experimental and in silico) approach. The far-UV circular dichroism (CD) spectrum of this peptide was evaluated both in aqueous and membrane mimicking solvents (TFE, HFIP and Dioxane). The results indicate that kiss 1 peptide adopted helical, turn and β conformations in membrane like environments. The near-UV CD spectroscopy was also carried out to examine the tertiary packing around aromatic residues of kiss 1 peptide and the peptide-membrane complex. The kiss 1 peptide exhibited little signal in water, but a prominent negative band was observed at around 275 nm when membrane mimetic solution was added. The observed ordered conformations of kiss 1 peptide in the different solvents indicated its potential biological activity which could enhance the secretion of gonadotropin-releasing hormone (GnRH) at BPG axis. The conformational information generated from the present study reinforces the application prospects of bioactive synthetic peptide analogs of kisspeptin 1 in improving the reproductive performances of important cultivable fish species.

Kisspeptins and their receptors in the brain-pituitary-gonadal axis of Odonthestes bonariensis: Their relationship with gametogenesis along the reproductive cycle

In vertebrates, the reproduction is controlled by the brain-pituitary-gonadal (BPG) axis and kisspeptin has emerged as a key player of this axis. In this study, we analyzed changes in the expression levels of kiss1, kiss2, and their receptors, kissr2 and kissr3 during gametogenesis in the BPG axis of feral Odontesthes bonariensis. In females, levels of brain kiss1 showed an increase at final maturation (Fm), while kiss2 levels were shown to be high at primary growth (Pg) stage, with no differences in the expression of their receptors. In the pituitary, kiss1 and kiss2 peaked at the cortical alveoli (Ca) stage, and kissr3 at initial vitellogenesis. In parallel, there was an increase of kiss1, kissr2 and kissr3 in the ovary during the Ca stage and both receptors again at Fm stage. In males, the four genes were highly expressed in the brain at the arrested (A) stage. In the pituitary, kiss2 peaked at spermatogonial (SG) and spermatocytary (SC) stages; while kissr3 reached a peak at the spermiogenic stage (SP). In testes, kiss1 and kiss2 significantly increased during the SG and SC stages; meanwhile, kissr2 increased at SG and SC, whereas kissr3 levels were significantly high at SC and SP stages. Taken together these results showed that the kisspeptin system in pejerrey is expressed in the three levels of the BPG axis with different expression profiles during the gonadal cycle. These findings pointed that kisspeptins have different roles in gametogenesis in this species.

Inhibitory action of tongue sole LPXRFa, the piscine ortholog of gonadotropin-inhibitory hormone, on the signaling pathway induced by tongue sole kisspeptin in COS-7 cells transfected with their cognate receptors

Kisspeptin (Kiss) acts as a positive regulator of reproduction by acting on gonadotropes and gonadotropin-releasing hormone (GnRH) neurons. Despite its functional significance, the intricate web of intracellular signal transduction pathways in response to Kiss is still far from being fully understood in teleosts. Accordingly, we investigated the molecular mechanism of Kiss action and its possible interaction with LPXRFa signaling in this study. In vitro functional analysis revealed that synthetic tongue sole Kiss2 decapeptide increased the cAMP responsive element-dependent luciferase (CRE-luc) activity in COS-7 cells transfected with its cognate receptor, while this stimulatory effect was markedly reduced by two inhibitors of the adenylate cyclase (AC)/protein kinase A (PKA) pathway. Similarly, Kiss2 also significantly stimulated serum responsive element-dependent luciferase (SRE-luc) activity, whereas this stimulatory effect was evidently attenuated by two inhibitors of the phospholipase C (PLC)/protein kinase C (PKC) pathway. In addition, LPXRFa-2 suppressed Kiss2-elicited CRE-luc activity in a dose-dependent manner. Taken together, Kiss2 utilizes both AC/PKA and PLC/PKC pathways to exert its functions via its cognate receptor and LPXRFa may antagonize the action of Kiss2 by inhibiting kisspeptin signaling. As far as we know, this study is the first to characterize the half-smooth tongue sole kisspeptin and LPXRFa signaling pathway in COS-7 cells transfected with their cognate receptors and provides novel information on the interaction between LPXRFa system and kisspeptin system in teleosts.

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.

Molecular characterization of Kiss2 receptor and in vitro effects of Kiss2 on reproduction-related gene expression in the hypothalamus of half-smooth tongue sole (Cynoglossus semilaevis)

Kisspeptin (Kiss) and its receptor, KissR (previously known as GPR54), play a critical role in the control of reproduction and puberty onset in mammals. Additionally, a number of studies have provided evidence of the existence of multiple Kiss/KissR systems in teleosts, but the physiological relevance and functions of these kisspeptin forms (Kiss1 and Kiss2) still remain to be investigated. To this end, we examined the direct actions of Kiss2 on hypothalamic functions in the half-smooth tongue sole (Cynoglossus semilaevis), a representative species of the order Pleuronectiformes. As a first step, the full-length cDNA for kiss2r was identified and kiss2r transcripts were shown to be widely expressed in various tissues, notably in the brain of tongue sole. Then, the effects of Kiss2 decapeptide on reproduction-related gene expression were evaluated using a primary hypothalamus culture system. Our results showed that neither gnrh2 nor gnrh3 mRNA levels were altered by Kiss2. However, Kiss2 significantly increased the amounts of gnih and kiss2 mRNAs. In contrast, Kiss2 elicited an evident inhibitory effect on both gnihr and kiss2r mRNA levels. To the best of our knowledge, this is the first description of a direct and differential regulation of reproduction-related gene expression by Kiss2 at the hypothalamus level of a teleost fish. Overall, this study provides novel information on the role of Kiss2/Kiss2R system in the reproductive function of teleosts.

Characterization of kiss2 and kissr2 genes and the regulation of kisspeptin on the HPG axis in Cynoglossus semilaevis

Reproduction allows organisms to produce offspring. Animals shift from immature juveniles into mature adults and become capable of sexual reproduction during puberty, which culminates in the first spermiation and sperm hydration or ovulation. Reproduction is closely related to the precise control of the hypothalamic-pituitary-gonadal (HPG) axis. Kisspeptin peptides are considered as the important regulator of HPG axis in mammalian. However, the current understanding of kisspeptin in flatfish is not comprehensive. In this study, we cloned and analyzed the kiss2 and kissr2 genes in Cynoglossus semilaevis. Interesting alternative splicing in the 5'-untranslated regions (UTR) of the Cskissr2 gene was found. The expression profiles of Cskiss2 and Cskissr2 showed relative high messenger RNA (mRNA) levels at the late gastrula stage during embryonic development, at total length = 40 mm during early gonadal differentiation, and in the brains and gonads of all investigated tissues. These results suggested that the kisspeptin system participated in embryogenesis and in the regulation of gonadal differentiation and development. Considering that the control and regulatory mechanisms of kisspeptin in the central reproductive axis are still unclear, we documented that the intramuscular injection of kisspeptin caused different sGnRH and cGnRH mRNA levels in a dose- and tissue-dependent manner. The mRNA expressions of FSH and LH were stimulated in the ovary and were inhibited in the testis under the kisspeptin treatments. These results provided foundations for understanding the roles of kisspeptin in the neuroendocrine system in fish. The manipulation of the kisspeptin system may provide new opportunities to control the gonadal development and even reproduction in fish.

Neurokinin B regulates reproduction via inhibition of kisspeptin in a teleost, the striped bass

Kisspeptin and neurokinin B (NKB) are neuropeptides co-expressed in the mammalian hypothalamus and coordinately control GnRH signaling. We have found that Nkb and kisspeptin neurons are distinct in the teleost, striped bass (STB) and capitalized on this phenomenon to study the mode of action of Nkb and its related neuropeptide-F (Nkf), both of which are encoded by the tac3 gene. In vitro brain slices and in vivo administration studies revealed that Nkb/f consistently downregulated kiss2, whereas antagonist (AntD) administration restored this effect. Overall, a minor effect was noted on gnrh1 expression, whereas Gnrh1 content in the pituitaries was reduced after Nkb/f treatment and increased with AntD. Concomitantly, immunostaining demonstrated that hypothalamic Nkb neurons border and densely innervate the largest kiss2 neuronal population in the hypothalamus, which also coexpresses Nkb receptor. No expression of Nkb receptor or Nkb neuronal projections was detected near/in Gnrh1 soma in the preoptic area. At the level of the pituitary, however, the picture was more complex: both Nkb/f and AntD upregulated lhb and fshb expression and Lh secretion in vivo Together with the stimulatory effect of Nkb/f on Lh/Fsh secretion from pituitary cells, in vitro, this may indicate an additional independent action of Nkb/f within the pituitary, in which the hypothalamic pathway is more dominant. The current study demonstrates that Nkb/f utilizes multiple pathways to regulate reproduction in the STB and that in the brain, Nkb mainly acts as a negative modulator of kiss2 to regulate the release of Gnrh1.

Characterization, molecular docking, dynamics simulation and metadynamics of kisspeptin receptor with kisspeptin

We report molecular characterization of the kisspeptin receptor (kiss1r), an essential gatekeeper for reproduction and onset of puberty in vertebrates. The full-length cDNA sequence of kiss1r is 1786bp which consist of 5' UTR (untranslated region) 261bp, 3' UTR of 424bp and open reading frame of 1101 encoding a putative protein of 366 amino acids. Basal tissue expression pattern of kiss1r mRNA revealed that it is mainly expressed in the brain and testis. We also report the structure of the kiss1r, along with plausible activation mechanism of this receptor by kisspeptin using computational modelling and dynamic simulation approach of multiple 100ns of timescale. A present modelling and simulations studies shed light on the molecular level of interaction, suggesting that direct hydrogen bonds between ASN4, SER5, GLY7, ARG9 and PHE10 of kisspeptin and TRP7, ASN8, GLU11, ILE17, ASN19 and TYR183 of kiss1r could be crucial role players in initial binding of receptor and the kisspeptin towards allosteric modulatory effects of kisspeptin on the receptor. To the best our knowledge, this is the first report on computational modelling and molecular dynamic simulations of kiss1r in animals shedding light on its possible mode of activation.

Photoperiod regulate gonad development via kisspeptin/kissr in hypothalamus and saccus vasculosus of Atlantic salmon (Salmo salar)

Atlantic salmon exhibit seasonal reproduction. However, the mechanisms governing this are still unclear. Generally speaking, kisspeptin has been recognized as a regulator of reproduction. Here, we report a relationship between kisspeptin, GnRH and photoperiod in Atlantic salmon. The results demonstrated that the expression of the Atlantic salmon kisspeptin-receptor (skissr) was not always consistent with the expression pattern of Atlantic salmon GnRH3 (sGnRH3) during all developmental processes. Kisspeptin may exert its influence primarily in the early and later stages of gonad development by promoting the secretion of sGnRH3. Meanwhile, the expression levels of kissr were higher in fish with gonads at stage II and stage V under the long-day photoperiod regime than under the short-day regime. In addition, both skissr and sGnRH3 were also expressed in the saccus vasculosus (SV), an organ only found in fish. The SV might be a seasonal sensor regulating reproduction in addition to the hypothalamus (Hyp).

Eel Kisspeptins: Identification, Functional Activity, and Inhibition on both Pituitary LH and GnRH Receptor Expression

The European eel (Anguilla anguilla) presents a blockade of sexual maturation at a prepubertal stage due to a deficient production of gonadotropins. We previously initiated, in the eel, the investigation of the kisspeptin system, one of the major gatekeepers of puberty in mammals, and we predicted the sequence of two Kiss genes. In the present study, we cloned and sequenced Kiss1 and Kiss2 cDNAs from the eel brain. The tissue distributions of Kiss1 and Kiss2 transcripts, as investigated by quantitative real-time PCR, showed that both genes are primarily expressed in the eel brain and pituitary. The two 10-residue long sequences characteristic of kisspeptin, eel Kp1(10) and Kp2(10), as well as two longer sequences, predicted as mature peptides, eel Kp1(15) and Kp2(12), were synthesized and functionally analyzed. Using rat Kiss1 receptor-transfected Chinese hamster ovary cells, we found that the four synthesized eel peptides were able to induce [Ca²⁺]_i responses, indicating their ability to bind mammalian KissR-1 and to activate second messenger pathways. In primary culture of eel pituitary cells, all four peptides were able to specifically and dose-dependently inhibit lhβ expression, without any effect on fshβ, confirming our previous data with heterologous kisspeptins. Furthermore, in this eel in vitro system, all four peptides inhibited the expression of the type 2 GnRH receptor (gnrh-r2). Our data revealed a dual inhibitory effect of homologous kisspeptins on both pituitary lhβ and gnrh-r2 expression in the European eel.