Temporal mRNA expression of transforming growth factor-beta superfamily members and inhibitors in the developing rainbow trout ovary

During mammalian ovarian development transforming growth factor-beta (TGFβ) superfamily members and their inhibitors are critical paracrine regulators, yet the intraovarian functions of these proteins have received less attention in fish. Using quantitative real-time RT-PCR, changes in ovarian mRNA expression of six TGFβ members and two inhibitors were investigated in rainbow trout across a wide range of fish ovarian stages (i.e., early perinucleous stage through acquisition of maturational competence). Transcript changes for insulin-like growth factor 1 and 2, and five enzymes associated with steroidogenesis, as well as plasma levels of three sex steroids were also measured to provide a framework of established intraovarian regulators in trout. Expression of bone morphogenetic protein 4 (bmp4), bone morphogenetic protein7 (bmp7), and growth differentiation factor 9 (gdf9) peaked during pre-vitellogenic stages and steadily decreased through advancing stages implicating these genes in early ovarian development. A dramatic increase in inhibin betaA and decrease in follistatin expression occurred during early to mid-vitellogenic stages, which corresponded with increased 17β-estradiol plasma levels suggesting a vitellogenic role for ovarian activin A. Follicles that were competent to respond to the maturation-inducing hormone had decreased levels of inhibin betaB and increased expression of bambi (bmp and activin membrane-bound inhibitor) suggesting their roles in maturation processes. Furthermore, bmp4, bmp7 and gdf9 are primarily expressed in the oocyte whereas the inhibin subunits, follistatin, and bambi are primarily expressed in the somatic follicle cells. These results support TGFβ superfamily members and their inhibitors have wide-ranging and disparate roles in regulating ovarian development in fish.