Conditional Fgfr1 Deletion in GnRH Neurons Leads to Minor Disruptions in the Reproductive Axis of Male and Female Mice

Abstract

In humans and mice, inactivating mutations in fibroblast growth factor receptor 1 (Fgfr1) lead to gonadotropin-releasing hormone (GnRH) deficiency and a host of downstream reproductive disorders. It was unclear if Fgfr1 signaling directly upon GnRH neurons critically drove the establishment of a functional GnRH system. To answer this question, we generated a mouse model with a conditional deletion of Fgfr1 in GnRH neurons using the Cre/loxP approach. These mice, called Fgfr1cKO mice, were examined along with control mice for their pubertal onset and a host of reproductive axis functions. Our results showed that Fgfr1cKO mice harbored no detectable defects in the GnRH system and pubertal onset, suffered only subtle changes in the pituitary function, but exhibited significantly disrupted testicular and ovarian morphology at 25 days of age, indicating impaired gametogenesis at a young age. However, these disruptions were transient and became undetectable in older mice. Our results suggest that Fgfr1 signaling directly on GnRH neurons supports, to some extent, the reproductive axis function in the period leading to the early phase of puberty, but is not critically required for pubertal onset or reproductive maintenance in sexually mature animals.

Keywords: Kallmann syndrome; conditional deletion; congenital hypogonadotropic hypogonadism; fibroblast growth factor receptor 1; gonadotropin-releasing hormone neurons; hypothalamic-pituitary gonadal axis.

Figure 1

Hypothalamic GnRH (A), pituitary FSH (B), pituitary LH (C), and serum LH (D) in PN25 and PN60 control and Fgfr1cKO male mice. Each bar represents mean ± SEM, n = 5–8. Two-way ANOVA results are indicated in Results section. Different letters above the bars indicate P < 0.05 by post-hoc test.

Figure 2

Hypothalamic GnRH (A), pituitary FSH (B), pituitary LH (C), and serum LH (D) in PN25 and PN60 (± 5 days) control and Fgfr1cKO female mice. All PN60 females were sacrificed on diestrus. Each bar represents mean ± SEM, n = 5–10. Two-way ANOVA results are indicated in Results section. Different letters above the bars indicate P < 0.05 by post-hoc test.

Figure 3

Relative expression of GnRH in PN25 and PN60 control and Fgfr1cKO males (A) and females (B). Each bar represents mean ± SEM, n = 5–9. No significant differences were observed between genotypes and age groups.

Figure 4

Testicular morphology in PN25 and PN60 control and Fgfr1cKO male mice. ST area (A), ST perimeter (B), %ST with open lumen (C), and % ST with mature spermatozoa (D) were scored by an investigator blind to the identity of the samples. Each bar represents mean ± SEM, n = 5-7. Two-way ANOVA results are indicated in Results section. Different letters above the bars indicate P < 0.05 by post-hoc test.

Figure 5

Representative photomicrographs of testicular morphology in PN25 and PN60 control and Fgfr1cKO males. For PN25 (A, B), thin arrows in the control testis (A) point to open ST with visible spermatozoa, and open arrows in the Fgfr1cKO testis (B) point to open ST with no visible spermatozoa. For PN60 (C, D), thick arrows point to ST with abundant mature spermatozoa in control (C) and Fgfr1cKO males (D). Scale bars = 100 µm.

Figure 6

Ovarian morphology in PN25 and PN60 control and Fgfr1cKO female mice. Total number of primordial follicles (A), preantral follicles (B), and antral follicles (C) at PN 25 and PN60 (± 5) and total number of corpora lutea (D) at PN60 (± 5) were assessed by an investigator blind to the identity of the samples. All PN60 females were sacrificed on diestrus. Each bar represents mean ± SEM, n = 5–7. Two-way ANOVA results are indicated in Results section. Different letters above the bars indicate P < 0.05 by post-hoc test.

Figure 7

Representative photomicrographs of ovarian morphology in PN25 (A, B) and PN60 (± 5 days; C, D) control (A, C) and Fgfr1cKO (B, D) females. All PN60 females were sacrificed on diestrus. Representative primordial follicles (blue box), preantral follicles (white box), antral follicles (green box), and corpus luteum (black circle) were indicated. Scale bars = 100 µm.