THE GH-IGF AXIS AND ITS POTENTIAL ROLE IN THE OVARY OF ZEBRAFISH, Danio rerio

Abstract

Growth hormone (GH) from the pituitary is the major hormone responsible for body growth in vertebrates. However, there has been increasing evidence that GH is also involved in several reproductive functions including steroidogenesis and gametogenesis in the gonads. In view of this, GH has also been termed co-gonadotropin. It is well established that most effects of GH are mediated by insulin-like growth factors (IGFs) produced in different tissues. In the ovary, however, both IGF-dependent and independent mechanisms have been proposed for GH actions, and one of the factors that work downstream of GH signaling appears to be activin. Interestingly, the expression of GH itself has been detected in the ovary, suggesting the existence of an intraovarian GH-IGF axis. Using zebrafish as the model, the present project was undertaken to characterize the mini-GH-IGF-I axis in the ovary with particular reference to its relationship with the local activin-follistatin system. RTPCR analysis showed that Gh (gh), Igf1 (igf1), Igf2 (igf2), and their receptors were all expressed in the ovary, indicating that a potential intraovarian GH-IGF system also exists in the zebrafish. Further experiment on spatial distribution using isolated follicle layers and denuded oocytes revealed that gh was exclusively expressed in the fullgrown oocytes, while its receptor Ghr (ghr) could be detected in both oocytes and follicle cells. Igf1 and its receptor igf1r as well as igf2 could also be detected in both oocytes and follicle cells. Analysis of temporal expression patterns demonstrated that gh and ghr were differentially expressed during folliculogenesis. The expression level of gh in the follicles seemed to be the highest at the primary growth (PG) stage, but it gradually decreased when PG follicles developed to full-grown (FG) follicles. On the contrary, the expression of ghr was low at the PG stage but significantly increased in the previtellogenic (PV) stage and remained high in later stages. Similarly, the expression of igf1r gradually increased from PG to mid-vitellogenic (MV) stage, but decreased significantly in the final FG stage. Igf1 expression increased from PG to EV stage and decreased afterwards till FG stage. While expression of igf2 maintained constant during folliculogenesis. When tested in cultured follicle cells, recombinant human Igf1 increased the expression of activin beta A (inhba) but decreased that of activin beta B (inhbb) in a clear time and dose-dependent manner. GH (bream and human GH), however, seemed to increase the expression of both inhba and inhbb, which supports our previous observation that GH-induced zebrafish oocyte maturation could be blocked by follistatin, an activin-binding protein. Although we have not obtained convincing evidence for IGFs as the downstream mediators of GH in the zebrafish ovary, our preliminary data did suggest a potential role for the local ovarian activin system in GH as well as IGF signaling. (platform)