The paracrine IGF-I system and its role in gonad maturation of sterlet, Acipenser ruthenus Linnaeus, 1758

The artificial reproduction of sturgeon is often affected by a stagnation of gonad development, particularly in females. Recent studies in teleosts revealed IGF-I mediated effects in follicle development in vitro, suggesting the involvement of paracrine IGF-I in gonad maturation. Therefore, the paracrine IGF-I system might be an important factor regulating gonad development, in particular the vitellogenesis of ovarian follicles in sturgeon. The complete coding sequence (CDS) of IGF-I and partial CDS of the corresponding IGF-I receptor (IGF-IR) were identified in sterlet, Acipenser ruthenus. A sensitive real-time PCR assay for the quantification of IGF-I and IGF-IR was established, normalising the expression of the target genes to a housekeeping gene (ß-microglobulin, ß2m). Comparing different tissues revealed a high expression of IGF-I and IGF-IR in the ovary: IGF-I mRNA level in the ovary was comparable to the liver and significantly higher than the expression rates found in the brain, the adipose tissue and the muscle. The expression of IGF-IR in the ovary was significantly higher than in the liver and the adipose tissue, and insignificantly higher in the muscle and the brain. In differentially developing cohorts of follicles, highest IGF-I and IGF-IR expression were found in vitellogenic follicles, whereas lowest expression rates of both target genes were detected in arrested, previtellogenic follicles. During a two-year period, the expression of the IGF-I system was correlated to the development of the female gonad, respectively. In contrast, a steady expression of IGF-I and IGF-IR was found in males. IGF-I mRNA levels were significantly lower in males throughout the reproductive cycle. When IGF-I and IGF-IR expression in maturing and stagnating females were compared, a 2.8-fold increase in IGF-I mRNA and a 2-fold increase in IGF-IR mRNA were detected in maturing sterlet females. The regulation of IGF-I and IGF-IR was studied in vitro in hepatocyte and follicle cell cultures: Among all hormones tested, cortisol exhibited the most significant effect on IGF-I mRNA expression in follicle cell cultures, whereas in hepatocyte primary cell cultures similar IGF-I induction was detected for GH and T3. None of the hormones applied here affected the expression of IGF-IR in vitro. These findings strongly suggest an important role of the paracrine IGF-I system in gonad maturation of female sterlet. It is concluded that IGF-I triggers the onset of vitellogenic growth phase, determining the faith of follicle development. The regulation of IGF-I expression by cortisol represents a potential target for aquaculture manipulation, where further studies are needed.