Regulation of membrane progestin receptors in the zebrafish ovary by gonadotropin, activin, TGF-β and BMP-15

Progestin hormones are vital for inducing oocyte maturation in fish by binding to membrane progestin receptors (mPRs). The aim of this study was to examine the expression and regulation of mPRα and mPRβ in zebrafish follicles. First, defolliculated fully grown oocytes were subjected to immunofluorescent staining using anti-mPRα and mPRβ antibodies, and their expression on the oocyte membrane was confirmed. Second, total protein was collected from zebrafish follicles and Western blotting revealed that the level of mPRα and mPRβ increased with follicle development. We have previously shown that several members of the transforming growth factor-β (TGF-β) superfamily, including TGF-β1, activin-A, and bone morphogenetic protein (BMP)-15, regulate oocyte maturation in zebrafish. Therefore, the third major focus of this study was to test if these growth factors, as well as gonadotropins, regulate the expression of mPRs. Overexpression of BMP-15 significantly reduced, while knockdown of BMP-15 increased, mPRβ levels. However, mPRα expression level remained unchanged with BMP-15 overexpression or knockdown. Treatment of follicles with human chorionic gonadotropin (hCG) resulted in an increased in mPRβ, but not mPRα, expression levels. Activin-A induced the expression of mPRα and mPRβ in a dose- and time-dependent manner. On the other hand, TGF-β1 treatment suppressed the expression of mPRβ, but not mPRα. Taken together, these findings further support the role of mPRs in oocyte maturation and suggest that gonadotropins, BMP-15, activin-A, and TGF-β1 exert their regulatory effects on oocyte maturation in part by regulating mPR expression.