Expression and Regulation of Gonadotropins (fshb, lhb) and Growth Hormone (gh) during Ovarian Differentiation and Pubertal Onset of Female Zebrafish

Being an estrogenic chemical and its ubiquitous presence in our daily lives, the effects of bisphenal A (BPA) on human health have received tremendous attention in recent years. Studies on the effects of BPA on female reproductive system have shown that early exposure to BPA during the prenatal or postnatal period impairs reproductive functions, including disruption of ovarian development, advanced pubertal onset, and the induction of an early, and persistent estrus. In teleost fish, few studies have been reported on the effects of BPA on reproductive function, except in medaka, Kryptolebias marmoratus, turbot, goldfish, common carp, brown trout and zebrafish. Despite these studies, the effects of BPA on reproductive axis remain largely unknown in teleost fish. On the other hand, in the zebrafish model, the major developmental events of reproduction, including the timing of puberty onset, the spatiotemporal expression patterns of key pituitary hormones such as GTHs (FSH and LH) during gonadal differentiation and puberty development remain largely unknown. Therefore, the information on these issues in zebrafish not only is valuable for understanding the roles of GTHs in early sexual development; also facilitate our study on the effects of BPA on the reproductive axis in the zebrafish.;Using histology analysis, double-colored fluorescent in situ hybridization (FISH), real-time qPCR, proteomic analysis and in vivo treatment, this study was undertaken to explore the timing of sex differentiation and puberty onset, the ontogenic expression patterns of FSH (fshb) and LH ( lhb) subunits in the zebrafish pituitary with particular emphasis on the stage of sexual differentiation and puberty onset, and the effects of BPA on the reproductive axis in zebrafish during prepubertal period. To define the timeline of sex differentiation and puberty onset, and the ontogenic expression profiles of GTHs, the zebrafish were collected at different time points from early development stage to sexual maturation. The gonadal developmental stage was analyzed by histological examination. For the expression profiles of GTHs, the head of each fish including the brain and pituitary was sampled for FISH analysis. To investigate the influence of BPA on the reproductive function, Juvenile zebrafish of 20 day post-fertilization (dpf) were exposed to BPA (10 microM) for 20 days followed by sampling the brain, pituitary, liver and ovary for histological, in situ hybridization, expression analyses and proteomic analyses at 40 dpf. 17beta-estradiol (E2, 10 nM) and testosterone (T, 10 nM) were also used as a positive control.;In female zebrafish, the first morphological sign for puberty is the first wave of follicle transition from the primary growth (PG) to previtellogenic stage (PV), our results showed that it occurs around 45 day post fertilization (dpf). Meanwhile, the puberty onset was highly depending on the somatic growth. The expression of fshb was much earlier than that of lhb with its mRNA signal detectable (∼2-3 cells/pituitary) shortly after hatching (4 dpf). In contrast, lhb expression became detectable at the time of sex differentiation (∼25 dpf). Interestingly, the number of lhb-expressing cells was very low (∼5-6 cells/pituitary) before puberty but increased dramatically during and after puberty onset. In contrast, the expression of fshb was abundant before puberty with only a slight increase in cell number during puberty onset. Our result strongly suggests an important role for LH at the puberty onset of female zebrafish. On the other hand, although BPA and E2 both promoted ovarian differentiation, they significantly suppressed the ovarian growth afterwards in the zebrafish. Meanwhile, both BPA and E2, but not T, dramatically decreased the expression of fshb in the pituitary, which was well correlated with the suppression of ovarian size. However, the expression of the upstream regulators of GTHs, including kiss1, kiss2, gnrh2 and gnrh3, was not affected by BPA and E2. Interestingly, at the liver level, BPA and E2 displayed different effects. E2 induced abdominal swelling due to a significant hepatic hyperplasia. However, BPA exposure had no such effect on the liver. These results indicate that BPA has estrogenic effects on female reproduction, but it does not mimic E2 in all aspects, at least for the dose tested in the present study.