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

Nitric oxide mediated kisspeptin regulation of steroidogenesis and gametogenesis in the catfish, Clarias batrachus

Nitric oxide (NO) is a gaseous molecule that regulates various reproductive functions. It is a well-recognized regulator of GnRH-FSH/LH-sex steroid secretion in vertebrates including fish. Kisspeptin is a recently discovered neuropeptide which also regulates GnRH secretion. Nitrergic and kisspeptin neurons are reported in close physical contact in the mammalian brain suggesting their interactive role in the release of GnRH. The existence of kisspeptin and NOS is also demonstrated in vertebrate gonads, but information on their reciprocal relation in gonads, if any, is obscure. Therefore, attempts were made to evaluate the functional reciprocal relation between nitric oxide and kisspeptin in the catfish gonads, if any, by administering the nitric oxide synthase (NOS) inhibitor, L-NAME {N(G)-nitro-L-arginine methyl ester}, which reduces NO production, and kisspeptin agonist (KP-10) and assessing their impacts on the expressions of kisspeptin1, different NOS isoforms, NO and steroid production in the gonadal tissue. The results revealed that L-NAME suppressed the expression of kiss1 in gonads of the catfish establishing the role of NO in kisspeptin expression. However, KP-10 increased the expression of all the isoforms of NOSs (iNOS, eNOS, nNOS) and concurrently NO and steroids in the ovary and testis. In vitro studies also indicate that kisspeptin stimulates the production of NO and estradiol and testosterone levels in the gonadal explants and medium. Thus, in vivo results clearly suggest a reciprocal interaction between kisspeptin and NO to regulate the gonadal activity of the catfish. The in vitro findings further substantiate our contention regarding the interactive role of kisspeptin and NO in gonadal steroidogenesis.

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.

Food deprivation differentially modulates gene expression of LPXRFa and kisspeptin systems in the brain-pituitary axis of half-smooth tongue sole (Cynoglossus semilaevis)

LPXRFa, also known as gonadotropin-inhibitory hormone (GnIH), and kisspeptin (Kiss) are two major hypothalamic peptides that modulate the reproductive axis of vertebrates, including teleosts. However, little information is available regarding the actions of nutritional status on the regulation of these two neuroendocrine systems in fish. Herein, we assessed the effects of starvation and refeeding on the expression of lpxrfa, kiss2 and their receptors (lpxrfa-r and kiss2r respectively) at the brain-pituitary level of half-smooth tongue sole (Cynoglossus semilaevis). Food deprivation for 4 weeks induced a rise in brain lpxrfa as well as brain and pituitary lpxrfa-r mRNA levels, and refeeding restored brain lpxrfa and lpxrfa-r expression back to normal. However, pituitary lpxrfa-r mRNA levels still remained high after 1 week of refeeding. Neither lpxrfa nor kiss2 transcripts in the pituitary were altered by fasting, but their mRNA levels increased significantly after 1 week of refeeding, and declined back to the control levels after 2 weeks of refeeding. None of brain kiss2 and kiss2r along with pituitary kiss2r transcripts were modified by the nutritional status. In summary, our results revealed an interaction between energy status and the elements of LPXRFa and Kiss systems in the brain-pituitary axis of half-smooth tongue sole. Food deprivation and refeeding differentially regulated the two systems, which provided additional evidence for the involvement of the LPXRFa and Kiss systems in the regulation of reproduction by energy balance in non-mammalian species.

The role of Kisspeptin signaling in Oocyte maturation

Kisspeptins (KPs) secreted from the hypothalamic KP neurons act on KP receptors (KPRs) in gonadotropin (GPN) releasing hormone (GnRH) neurons to produce GnRH. GnRH acts on pituitary gonadotrophs to induce secretion of GPNs, namely follicle stimulating hormone (FSH) and luteinizing hormone (LH), which are essential for ovarian follicle development, oocyte maturation and ovulation. Thus, hypothalamic KPs regulate oocyte maturation indirectly through GPNs. KPs and KPRs are also expressed in the ovarian follicles across species. Recent studies demonstrated that intraovarian KPs also act directly on the KPRs expressed in oocytes to promote oocyte maturation and ovulation. In this review article, we have summarized published reports on the role of hypothalamic and ovarian KP-signaling in oocyte maturation. Gonadal steroid hormones regulate KP secretion from hypothalamic KP neurons, which in turn induces GPN secretion from the hypothalamic-pituitary (HP) axis. On the other hand, GPNs secreted from the HP axis act on the granulosa cells (GCs) and upregulate the expression of ovarian KPs. While KPs are expressed predominantly in the GCs, the KPRs are in the oocytes. Expression of KPs in the ovaries increases with the progression of the estrous cycle and peaks during the preovulatory GPN surge. Intrafollicular KP levels in the ovaries rise with the advancement of developmental stages. Moreover, loss of KPRs in oocytes in mice leads to failure of oocyte maturation and ovulation similar to that of premature ovarian insufficiency (POI). These findings suggest that GC-derived KPs may act on the KPRs in oocytes during their preovulatory maturation. In addition to the intraovarian role of KP-signaling in oocyte maturation, in vivo, a direct role of KP has been identified during in vitro maturation of sheep, porcine, and rat oocytes. KP-stimulation of rat oocytes, in vitro, resulted in Ca2+ release and activation of the mitogen-activated protein kinase, extracellular signal-regulated kinase 1 and 2. In vitro treatment of rat or porcine oocytes with KPs upregulated messenger RNA levels of the factors that favor oocyte maturation. In clinical trials, human KP-54 has also been administered successfully to patients undergoing assisted reproductive technologies (ARTs) for increasing oocyte maturation. Exogenous KPs can induce GPN secretion from hypothalamus; however, the possibility of direct KP action on the oocytes cannot be excluded. Understanding the direct in vivo and in vitro roles of KP-signaling in oocyte maturation will help in developing novel KP-based ARTs.

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.

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.