Leptin Signaling on Gonadotropes Is Crucial for Normal Cyclicity and Ovarian Follicular Development: Associations with Pituitary Estrogen Receptor Expression and Reproductive Capacity

Presentation Number: SUN 495
Date of Presentation: April 2nd, 2017

Angela K. Odle*, Andrea Melgar Castillo, Melody Lyn Allensworth-James, Anessa C. Haney, Angus M. MacNicol, Melanie C. MacNicol and Gwen V. Childs
University of Arkansas for Medical Sciences, Little Rock, AR

Abstract

Nutrition and reproduction are highly integrated. Circulating levels of the adipokine leptin reflect white fat content, and leptin is a crucial nutritional signal for reproduction. We have previously shown that deletion of leptin receptors on gonadotropes leads to decreased expression of gonadotropin-releasing hormone receptor (GnRHr) proteins1. We hypothesized that leptin is crucial for the GnRHr protein expression levels needed to mount an LH surge, and that the loss of the leptin signal will cause changes in ovarian function and feedback to the pituitary.

We created a new model of gonadotrope leptin resistance, using the Cre-GnRHr mouse line crossed with the floxed leptin receptor exon 1 line. Mutant (MUT) animals from this line carry one allele of Cre-GnRHr and two alleles of floxed Lepr exon 1 (Gonadotrope-Lepr1-null). Controls (CTL) are Cre-negative with two alleles of floxed Lepr exon 1. All experiments were performed on female mice between 2 and 3 months of age. Cycling and breeding studies were performed on Gonadotrope-Lepr1-null females. Proestrus (PRO) and diestrus (DIE) female mutants and controls were killed at 0900 for collection of blood (EIAs), pituitaries (EIAs or qPCR), and uteri/ovaries (sectioning and staining). Statistical analysis was performed using Student’s t tests or ANOVA/Fisher’s LSD.

The levels of pituitary estrogen receptor-alpha (ERα) proteins were reduced in PRO mutants from 26 ng/mL ± 1.4 to 21.69 ng/mL ± 1 (n=5,5 p=0.02), and mRNA was reduced by 40% (p=.025). DIE mutant females have a dramatic decrease in serum prolactin (CTL: 5265 pg/mL ± 1794, MUT: 2091 ± 389, n=4,4, p=.047). As previously reported, serum LH was reduced to half of control levels in mutant PRO females (CTL: 940 pg/mL ± 368 MUT: 417.8 pg/mL ± 68.29, n=5,6, p=0.01).

The reduced serum LH correlated with a significant 33 or 38% reduction in average numbers of corpora lutea/ovary section (p<0.05) and a 40 or 47% decrease in antral follicles/ovary section (p<0.005) in PRO or DIE mutant mice, respectively. Serum estrogen was not changed, but serum progesterone was decreased in PRO mutants (CTL: 441.6 ± 8 ng/ml ; MUT: 309 ± 50 ng/ml n=5,6; p=0.03). Gonadotrope-Lepr1-null females spent significantly more time in PRO than littermate controls (p<0.05). As a consequence, mutant female breeders spend an average of 27 days between litters. Two of the mutant breeders had a decreased average number of pups per litter (6) compared to our normal 9-10 seen in the other breeders.

We have previously shown that the gonadotrope leptin signal is crucial for proper expression of GnRHr proteins. Here we show downstream effects leading to suboptimal ovarian function and delayed pre-ovulatory events. Ongoing studies are testing the hypothesis that the decrease in ERα and GnRHr expression causes the mutant to fail to mount an LH surge, which delays progression to estrus and causes the delayed fertility phenotype.

 

Nothing to Disclose: AKO, AM, MLA, ACH, AMM, MCM, GVC