Physiological Roles of Gonadotropin-Inhibitory Hormone Signaling in the Control of Mammalian Reproductive Axis: Studies in the NPFF1 Receptor Null Mouse

Abstract

RF-amide-related peptide-3 (RFRP-3), the mammalian ortholog of the avian gonadotropin-inhibiting hormone (GnIH), operates via the NPFF1 receptor (NPFF1R) to repress the reproductive axis, therefore acting as counterpart of the excitatory RF-amide peptide, kisspeptin (ligand of Gpr54). In addition, RFRP-3 modulates feeding and might contribute to the integrative control of energy homeostasis and reproduction. Yet, the experimental evidence supporting these putative functions is mostly indirect, and the physiological roles of RFRP-3 remain debatable and obscured by the lack of proper analytical tools and models. To circumvent these limitations, we characterize herein the first mouse line with constitutive inactivation of NPFF1R. Ablation of NPFF1R did not compromise fertility; rather, litters from NPFF1R null mice were larger than those from wild-type animals. Pubertal timing was not altered in NPFF1R deficient mice; yet, pre-pubertal knockout (KO) males displayed elevated LH levels, which normalized after puberty. Adult NPFF1R null male mice showed increased Kiss1 expression in the hypothalamic arcuate nucleus, higher serum FSH levels, and enhanced LH responses to GnRH. However, genetic elimination of NPFF1R was unable to reverse the state of hypogonadism caused by the lack of kisspeptin signaling, as revealed by double NPFF1R/Gpr54 KO mice. NPFF1R null mice displayed altered feedback responses to gonadal hormone withdrawal. In addition, metabolic challenges causing gonadotropin suppression, such as short-term fasting and high-fat diet, were less effective in dampening LH secretion in NPFF1R-deficient male mice, suggesting that absence of this inhibitory pathway partially prevented gonadotropin suppression by metabolic stress. Our data are the first to document the impact of elimination of GnIH signaling on reproductive parameters and their modulation by metabolic challenges. Whereas, in keeping with its inhibitory role, the NPFF1R pathway seems dispensable for preserved puberty and fertility, our results surface different alterations due to the lack of GnIH signaling that prominently include changes in the sensitivity to fasting- and obesity-associated hypogonadotropism.

RFamide-related peptide-3 (RFRP-3) suppresses sexual maturation in a eusocial mammal

“…In contrast to the small proportion of Kiss1/GPR147 coexpressing cells in other areas of the rodent brain, 25% of kisspeptin somata in the mouse Arc express GPR147 or GPR74, whereas 35% receive contact from RFRP fibers (66). Further, RFRP KO mice show higher numbers of Kiss1 positive neurons and increased total Kiss1 gene expression, specifically in the Arc (35). Hence, although kisspeptin neurons do not appear to signal to RFRP neurons (66), RFRP-3 may directly modulate a subset of hypothalamic kisspeptin cells in conventional laboratory rodents.…”

“…Our data bolster reports that the RFRP system is active developmentally and so challenge conclusions of congenital knockdown studies. Indeed, although disruption of RFRP-3 signaling fails to alter puberty onset in mice (34,35), it is presumptuous to rule out a role for RFRP-3 in reproductive development entirely as knockdown triggers a degree of prepubertal hyperactivity (35). Cross-species comparisons will determine whether exceptional features of the reproductive system underlie RFRP-3-mediated puberty delay in NMR subordinates and other hypogonadotrophic adults (e.g., subordinate marmosets), and so advance research on neural correlates of environmentally induced HPG dysfunction, which has thus far relied heavily on seasonally breeding species.…”

“…What limited developmental research exists is heterogeneous. Whereas knockdown of production and receptors for RFRP-3 does not appear to alter pubertal development in peripubertal mice (34,35), RFRP-3-aided inhibition of gonadotropin secretion has been observed in prepubertal animals (36), confirming bioactivity of this peptide at younger nonadult ages. These findings point to a potential role for RFRP-3 in maintaining the prepubertal reproductive axis in a quiescent state until environmental conditions are permissive for breeding.…”

Molecular, cellular, morphological, physiological and behavioral aspects of gonadotropin-inhibitory hormone

“…They determined that approximately 25% of GnIH neurons coexpress ERα in each sex (Table 1), whereas GnIH cells do not express androgen receptor in either sex, regardless of hormonal milieu . León et al (2014) characterized the first mouse line with constitutive inactivation of GPR147. Ablation of GPR147 did not compromise fertility; rather, litters from GPR147 null mice were larger than those from wild-type animals.…”

“…Pubertal timing was not altered in GPR147 deficient mice; yet, pre-pubertal knockout males displayed elevated LH levels (Table 1), which normalized after puberty. Adult GPR147 null male mice showed increased Kiss1 expression in the hypothalamic arcuate nucleus, higher serum FSH levels, and enhanced LH responses to GnRH (León et al, 2014) (Table 1). Yano et al (2004) investigated developmental changes in the expressions of GnIH mRNA and GnIH-ir neuronal cells and fibers in the rat brain.…”