8001 Effects of Gonadotrope-Specific Knockout of the Transcription Factors, Icer and Creb, on HPG Axis Function in Female Mice

Abstract Disclosure: C. Gabriel Meireles: None. N.P. Boris: None. K.N. Hausken: None. S.A. Pereira: None. G.A. Stamatiades: None. H. Kim: None. R.S. Carroll: None. U.B. Kaiser: None. Background and Objective: FSH and LH secretion from pituitary gonadotropes is controlled by GnRH, central to the regulation of reproduction. Released in a pulsatile manner from the hypothalamus, varying GnRH pulse frequencies and amplitudes alter FSH and LH secretion patterns. Notably, low vs. high GnRH pulse frequencies preferentially increase Fshb and Lhb expression, respectively, potentially via differential induction of transcription factors such as Icer and Creb. In vitro, Creb phosphorylation and Fshb expression are increased to a greater extent at low rather than high GnRH pulse frequencies; in contrast, Icer, which attenuates GnRH stimulation of Fshb expression by antagonizing Creb binding to the Fshb promoter, is induced to a greater extent at higher GnRH pulse frequencies. The mechanisms driving differential responses to pulsatile GnRH in vivo have not yet been fully elucidated. The goal of this study was to determine the roles of Creb and Icer in gonadotropin regulation in vivo in mice. We hypothesized that gonadotrope-specific deletion of Creb or Icer would influence Fshb regulation and, consequently, reproduction and fertility. Methods and Results: Two mouse models, with gonadotrope-specific deletion of Creb (Creb KO) or Icer (Icer KO) were generated by crossing GnrhrGRIC/+ mice with Creb1fl/fl or Icerfl/fl mice, respectively. Ovarian morphology and gonadotrope response to GnRH stimulation and ovariectomy (OVX) were evaluated in Creb KO and Icer KO mice, compared to their controls (Creb1fl/fl and Icerfl/fl). We showed previously that Icer KO female have normal pubertal development but spend more time in diestrus/metestrus during the estrous cycle. In the present study, we found significantly more early antral follicles in Icer KO mice (p=0.04); in addition, diestrus Icer KO females had greater GnRH-induced LH secretion (but similar GnRH-induced FSH levels), compared to controls. Serum FSH levels were higher in intact Creb KO (p=0.01), but not in Icer KO mice (p=0.87), compared to controls. Interestingly, 14 days after OVX, female Icer KO mice had higher serum FSH levels (p=0.04) and pituitary Lhb mRNA levels, compared to controls (p=0.004). No differences were observed in Fshb mRNA levels in either Creb KO or Icer KO mice, compared to controls. Creb KO females were fertile and had normal timing for first litter, interval between litters and number of pups in fertility tests. Conclusions: Deletion of Icer from gonadotrophs in mice increased serum FSH levels, increased GnRH-stimulated LH secretion, and increased pituitary Lhb mRNA levels post-OVX, likely contributing to the increase in ovarian early antral follicles. In Creb KO mice, baseline serum FSH levels were higher than in controls. These findings suggest roles for both Icer and Creb in the regulation of LH and FSH and, consequently, in reproduction. Presentation: 6/2/2024