Early Prepubertal Ontogeny of Pulsatile Gonadotropin-Releasing Hormone (GnRH) Secretion: I. Inhibitory Autofeedback Control through Prolyl Endopeptidase Degradation of GnRH1

Abstract

GnRH[1-5], a subproduct resulting from degradation of GnRH by prolyl endopeptidase (PEP) and endopeptidase 24.15 (EP24.15) was known to account for an inhibitory autofeedback of GnRH secretion through an effect at the N-methyl-D-aspartate (NMDA) receptors. This study aimed at determining the possible role of such a mechanism in the early developmental changes in frequency of pulsatile GnRH secretion. Using retrochiasmatic explants from fetal male rats (day 20-21 of gestation), no GnRH pulses could be observed in vitro, whereas pulses occurred at a mean interval of 86 min from the day of birth onwards. This interval decreased steadily until day 25 (39 min), during the period preceding the onset of puberty. Based on GnRH[1-10] or GnRH[1-9] degradation and GnRH[1-5] generation after incubation with hypothalamic extracts, EP24.15 activity did not change with age, whereas PEP activity was maximal at days 5-10 and decreased subsequently until day 50. These changes were consistent with the ontogenetic variations in PEP messenger RNAs (mRNAs) quantitated using RT-PCR. Using fetal explants, the NMDA-evoked release of GnRH was potentiated in a dose-dependent manner by bacitracin, a competitive PEP inhibitor and the desensitization to the NMDA effect was prevented using 2 mM of bacitracin. At day 5, a higher bacitracin concentration of 20 mM was required for a similar effect. Pulsatile GnRH secretion from fetal explants was not caused to occur using bacitracin or Fmoc-Prolyl-Pyrrolidine-2-nitrile (Fmoc-Pro-PyrrCN), a noncompetitive PEP inhibitor. At postnatal days 5 and 15, a significant acceleration of pulsatility was obtained using 1 microM of Fmoc-Pro-PyrrCN or 2 mM of bacitracin. At 25 and 50 days, a lower bacitracin concentration of 20 microM was effective as well in increasing the frequency of GnRH pulsatility. We conclude that the GnRH inhibitory autofeedback resulting from degradation of the peptide is operational in the fetal hypothalamus but does not explain the absence of pulsatile GnRH secretion at that early age. After birth, PEP activity is high and may account for the low frequency of pulsatility. The potency of that effect decreases before the onset of puberty and may contribute to the acceleration of GnRH pulsatility.

Accelerating Onset of Puberty Through Modification of Early Life Nutrition Induces Modest but Persistent Changes in Bull Sperm DNA Methylation Profiles Post-puberty

“…Indeed, our study points to genes directly acting at different levels of this axis, such as PREP, WWOX, and CALB2. PREP encodes a prolyl endopeptidase that metabolizes gonadotropin-releasing hormone (GnRH) in the hypothalamus and pituitary gland and could contribute to pulsatile GnRH secretion that precedes the onset of puberty (Yamanaka et al, 1999). The PREP gene product is detected in both somatic cells and germ cells of the testis, as well as in sperm; PREP-deficient mice exhibit smaller testes, abnormal spermatogenesis and decreased sperm motility (Dotolo et al, 2016).…”

Roles of Purinergic P2X Receptors as Pacemaking Channels and Modulators of Calcium-Mobilizing Pathway in Pituitary Gonadotrophs

“…Experiments were performed in cells at holding potential of Ϫ60 mV, except for experiments with RB2, which were done at holding potential of Ϫ80 mV. and the washout/degradation of intrapituitary GnRH (52). Potentiation of GnRH-induced secretion by pituitary-derived ATP might also represent a positive feedback mechanism by which hormone secretion is stimulated without interfering with the pulsatile GnRH release.…”

Thimet Oligopeptidase (EC 3.4.24.15) Key Functions Suggested by Knockout Mice Phenotype Characterization

“…A possible role of brain THOP1 in the regulation of GnRH effects was suggested, for example, since intracerebroventricular administration of N -[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-p-aminobenzoate (cFP-AAF-pAB), a specific inhibitor of THOP1, resulted in increased gonadotropin secretion and increased recovery of intracerebroventricular-administered GnRH [4,14]. The direct correlation between THOP1 and in vivo GnRH metabolism has not been established here due to technical difficulties for GnRH and luteinizing hormone (LH) quantification in samples of portal hypophyseal and plasma from mice; these limitations have also been previously reported [59,60]. However, the lack of changes in the estrous cycle together with a normal reproductive function and litters of normal numbers, as well as normal expression of POP in all brain regions investigated herein, are strong evidences that both GnRH and LH levels are in a regular physiological range in THOP1 -/- .…”

“…Their litters had a normal number of animals, which cannot be visually distinguished from WT littermates. GnRH is critical for pubertal development and maintenance of reproductive competence, with normal estrous cycle being produced by a series of hormonal signals that starts with the release of GnRH from the hypothalamus [59,60]. After being secreted, GnRH can be degraded in the hypothalamus and the anterior pituitary gland by two endopeptidases—THOP1 and POP—acting in a stepwise manner [61].…”

“…Here we have also observed that POP mRNA expression was normal in ST, HC and PFC, which could be an additional explanation that THOP1 -/- have normal reproductive functions. POP is well known as a key enzyme for GnRH metabolism [61] and pulsatility [60], and could metabolize GnRH. Future investigations could be performed to further investigate the possible involvement of THOP1 in fine molecular tuning mechanisms related to sexual organ development, maturation and puberty onset.…”