Characterization and regulation of the insulin-like growth factor (IGF) system in the zebrafish (Danio rerio) ovary

Locally produced peptide hormones play an important role in the paracrine/autocrine regulation of ovarian development. The insulin-like growth factor (IGF) system is one family of local factors that has not been well studied in the ovary of fish. This study characterized the zebrafish (Danio rerio) ovarian IGF system, its spatial and temporal expression and regulation by gonadotropins and steroids. Three ligands (igf2a, igf2b, igf3) and two receptors (igf1ra and igf1rb) were demonstrated in the ovary using RT-qPCR. Though it was examined, igf1 expression was not detected in the zebrafish ovary. Igf3 expression significantly decreased in the hours prior to ovulation and was confined to the follicle cells. Igf2a, igf2b and the two receptors were detected in both the follicle cells and the oocyte and were constitutively expressed in ovarian tissue across the daily ovulatory cycle. In vitro addition of human chorionic gonadotropin (hCG; 20 IU/ml) stimulated a significant increase in igf3 expression in both midvitellogenic (MV; 0.45-0.56 mm) and full grown (FG; 0.57-0.65 mm) follicles while igf2b expression increased only in FG follicles. Treatment of follicles in vitro with 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P; 10 ng/ml) significantly decreased igf3 and igf2b expression in both MV and FG follicles. 17β-Estradiol (E₂; 25 ng/ml) had no effect on the expression of igf3 in MV or FG follicles. Igf1rb expression did not change after treatment with hCG, 17,20β-P or E₂. Collectively, these results demonstrate the presence of an ovarian IGF system in zebrafish that is differentially regulated by gonadotropin and steroids.