Kisspeptin Signaling in Rodents: Insights from Knock-Out Mice.William H. Colledge, Ph.D.

Abstract

Initiation of gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus at puberty stimulates luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release from the anterior pituitary to galvanize maturation of the gonads. A key molecular event that initiates this activation of GnRH secretion has recently been identified with the discovery that kisspeptin neuropeptides are potent agonists of GnRH release. Kisspeptins are a family of overlapping amidated peptides encoded by the Kiss1 gene that signal through the G-protein coupled receptor GPR54 (KISS1R). Five transgenic mouse lines have been generated with mutations in the Gpr54 gene and two lines with mutations in the Kiss1 gene. The phenotypes of these mutant mice are broadly similar, suggesting no obvious functional compensation by other ligands or receptors. The mutant mice fail to undergo puberty and have poor gonadal development and low sex steroid and gonadotropic hormone levels (hypogonadotropic hypogonadism). Spermatogenesis and ovulation are severely impaired and mutant females do not show normal estrous cyclicity. The gonads and the anterior pituitary retain functional responses to hormonal stimulation, however, consistent with the primary defect being a failure to secrete GnRH from the hypothalamus to activate the hypothalamic-pituitary-gonadal axis. These mutant mice are being used to interrogate the function of GPR54 and kisspeptins in key aspects of kisspeptin signaling including the role of these proteins in regulating the neuroendocrine responses of GnRH neurons. Kisspeptin expression is regulated by gonadal steroids (testosterone or estrogen) and this provides a possible mechanism by which these steroid hormones control GnRH secretion. We have shown that kisspeptin-GPR54 signaling is essential for GnRH neuronal activation and the pre-ovulatory LH surge. Using an ovariectomized, gonadal steroid-replacement regimen, which reliably generates an LH surge, 30% of kisspeptin neurons express c-FOS in surging mice compared with 0% in non-surging controls. A strong correlation was found between the percentage of c-FOS positive kisspeptin neurons and the percentage of c-FOS-positive GnRH neurons. To evaluate whether kisspeptin and/or GPR54 are essential for GnRH neuron activation and the LH surge, Gpr54 and Kiss1 null mice were examined. Wild-type mice all exhibited LH surges and c-FOS in ~50% of their GnRH neurons, but none of the mutant mice from either line showed an LH surge or any GnRH neurons with c-FOS. These observations provide the first evidence that kisspeptin-GPR54 signaling is essential for GnRH neuron activation that initiates ovulation. We have also shown that kisspeptin can stimulate GnRH secretion by a direct action at GnRH nerve terminals in addition to its likely action at the cell bodies. These mutant mice provide a useful resource to further understand the hypothalamic regulation of mammalian reproduction, its integration with the pituitary-gonadal axis and to study the potential function of Gpr54 and Kiss1 in peripheral tissues. This research was funded by Takeda Cambridge and BBSRC Grant BB/C003861/1.