Selective Deletion of Leptin Receptors in Gonadotropes Reveals Activin and GnRH-Binding Sites as Leptin Targets in Support of Fertility

Abstract

The adipokine, leptin (LEP), is a hormonal gateway, signaling energy stores to appetite-regulatory neurons, permitting reproduction when stores are sufficient. Dual-labeling for LEP receptors (LEPRs) and gonadotropins or GH revealed a 2-fold increase in LEPR during proestrus, some of which was seen in LH gonadotropes. We therefore investigated LEPR functions in gonadotropes with Cre-LoxP technology, deleting the signaling domain of the LEPR (Lepr-exon 17) with Cre-recombinase driven by the rat LH-β promoter (Lhβ-cre). Selectivity of the deletion was validated by organ genotyping and lack of LEPR and responses to LEP by mutant gonadotropes. The mutation had no impact on growth, body weight, the timing of puberty, or pregnancy. Mutant females took 36% longer to produce their first litter and had 50% fewer pups/litter. When the broad impact of the loss of gonadotrope LEPR on all pituitary hormones was studied, mutant diestrous females had reduced serum levels of LH (40%), FSH (70%), and GH (54%) and mRNA levels of Fshβ (59%) and inhibin/activin β A and β B (25%). Mutant males had reduced serum levels of GH (74%), TSH (31%), and prolactin (69%) and mRNA levels of Gh (31%), Ghrhr (30%), Fshβ (22%), and glycoprotein α-subunit (Cga) (22%). Serum levels of LEP and ACTH and mRNA levels of Gnrhr were unchanged. However, binding to GnRH receptors was reduced in LEPR-null LH or FSH gonadotropes by 82% or 89%, respectively, in females (P < .0001) and 27% or 53%, respectively, in males (P < .03). This correlated with reductions in GnRH receptor protein immunolabeling, suggesting that LEP's actions may be posttranscriptional. Collectively, these studies highlight the importance of LEP to gonadotropes with GnRH-binding sites and activin as potential targets. LEP may modulate population growth, adjusting the number of offspring to the availability of food supplies.

Leptin receptor null mice with reexpression of LepR in GnRHR expressing cells display elevated FSH levels but remain in a prepubertal state

“…The use of different approaches may explain the differences between the studies. Nevertheless, the results are complementary and reinforce the findings that leptin action in gonadotropes exerts a primary effect in adult, not prepubertal, mice and has a role in FSH secretion (1). Further studies using different models will be necessary to validate our findings and assess the mechanisms associated with the increase in FSH levels.…”

“…Conditional deletion of LepR in cells expressing LH-␤ delayed first pregnancy and decreased the number of pups per litter (1). The mechanisms underlying these effects are not completely understood, but the authors propose that the lack of leptin signaling altered GnRH binding sites and secretion of pituitary hormones.…”

“…Using our genetic approach, we were unable to test this hypothesis due to lack of pubertal development. However, the increase in FSH levels in the GRIC-LepR loxTB mice suggests that leptin action in gonadotropes may potentiate gonadotropin release (1). Previous studies have shown the majority of isolated gonadotropes express leptin-induced pSTAT3, and that mice with lack of leptin signaling in gonadotropes have decreased FSH levels (1).…”

“…the hypothalamus, the anterior pituitary gland, and the gonads (1,21,42,45,53). Thus leptin might control reproductive physiology by acting at different levels of the HPG axis.…”

“…However, recent findings showing that lack of leptin action only in LH-␤ϩ-gonadotropes also impairs fertility have reinvigorated the field. Female mice with deletion of LepR in luteinizing hormone ␤-subunit expressing cells (LH-␤, which is exclusively expressed in a subset of gonadotropes) display subfertility demonstrated by increased time to pregnancy and decreased litter size (1).…”