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Conclusion
Author statement
Acknowledgments
Introduction The hypothalamic-pituitary-gonadal (HPG) axis is an important neurohormone network for vertebrates that regulates their gonadal development and maturation, guarantees their pubertal development and maintains their normal reproduction. Gonadotropin-releasing hormone (GnRH) is the most important molecule for signal regulation in the HPG axis. GnRH is synthesized by intrahypothalamic secretory neurons and is pulsed through the HPG axis to the pituitary gland. It is the increase in pulsatile GnRH release which signals the onset of puberty (Titi et al., 2010; Javed et al., 2015). The main function of GnRH is to stimulate the anterior pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH), which activate the gonads to synthesize sex hormones, thus regulating reproductive organ development (Clarke and Cummins, 1982). Therefore, GnRH is at the center of the neuroendocrine network across the HPG axis and plays a crucial role in regulating the activities of other components along the axis and in maintaining the normal reproductive 2hcl in animals. The kisspeptin/G protein-coupled receptor 54 (GPR54) signaling system has been a focus of research on reproductive control for the promotion of animal reproductive efficiency. Kisspeptin is the peptide product of the Kisspeptin-1 (Kiss-1) gene, an endogenous agonist for GPR54 and an important upstream element regulating GnRH neurons (Skorupskaite et al., 2014; Novaira et al., 2014). Injection of kisspeptin into immature mice can induce precocious puberty, but this effect does not occur in mice without GPR54. Kisspeptin can also induce the release of LH in mice with Kiss-1 gene deletion (Seminara and Kaiser, 2005). Kisspeptin administered by direct ventricular injection or perfusion can induce LH and FSH expression in mice in a dose-dependent manner (Navarro et al., 2005). Moreover, kisspeptin can induce mouse hypothalami cultured in vitro to release GnRH (Thompson et al., 2010). In a GnRH secretory neuron cell line, kisspeptin treatment increased the secretion of GnRH and the expression of GnRH mRNA in a dose- and time-dependent manner (Novaira et al., 2009), while the GnRH antagonist acyline has been found to inhibit the release of gonadotropin stimulated by kisspeptin in primates (Shahab et al., 2005). Therefore, kisspeptin neurons are the central unit that regulates GnRH secretion. However, what is the molecular mechanism by which kisspeptin regulates GnRH expression? Studies on GnRH secretory cell lines and transgenic mice have shown that neuron-specific elements and enhancer elements in the promoter region of GnRH interact, playing an important role in GnRH expression (Kim et al., 2007). Further studies have shown that elements involved in the downstream response to kisspeptin and a binding site for the transcription factor orthodenticle homeobox 2 (Otx-2) are present in the promoter region of the mouse GnRH gene. Kisspeptin treatment can trigger increases in Otx-2 mRNA and protein expression and Otx-2 binding with the kisspeptin response element (KsRE) (Novaira et al., 2012). Previous studies on GnRH promoters in human, tilapia, zebrafish and snapper species have found many transcription factors bound to the GnRH promoter regions: CCAAT enhancer binding proteins (C/EBPs), Otx-2, organic cation transporter 1 (Oct-1) and SCIP/Tst-1 (POU domain transcription factor Oct-6) (Kelley et al., 2000; Eraly et al., 1998; Belsham and Mellon, 2000). In sheep, neurons that express Kiss-1 are located in the arcuate nucleus (ARC) of the hypothalamus and in the dorsolateral preoptic area (POA). In breeding seasons, the expression of kisspeptin in the ARC increases significantly, and the number of kisspeptin nerve fibers approaching GnRH neurons increases. In nonbreeding seasons, perfusion or intravenous injection of exogenous kisspeptin can promote the expression of Kiss-1 and stimulate the secretion of GnRH and LH, thus activating ovarian functions, facilitating estrogen secretion, and causing ovulation in ewes (Clarke et al., 2009; Clarkson and Herbison, 2006). Caraty et al. found that >80% of ewes in anestrus that were injected with kisspeptin ovulated, while only up to 20% of ewes ovulated without the injection, indicating that kisspeptin is a key factor affecting the start of seasonal reproduction in sheep (Caraty et al., 2007). The kisspeptin neurons in the ARC region of ewes mediate the negative and positive feedback control of estrogen over GnRH secretion, while the kisspeptin neurons in the POA region are involved only in the positive feedback mechanism and play a key role in sharply increasing LH before the induction of ovulation (Weems et al., 2015). A previous study has suggested that such plasticity is an important determinant of the rate of secretion of GnRH in animals of different metabolic status (Pouchain et al., 2017).