Novel Expression of Gonadotropin Subunit Genes in Oocytes of the Gilthead Seabream (Sparus aurata)

It is widely believed that FSH and LH, which are known to play key roles in controlling the production of functional oocytes in vertebrates, are synthesized and secreted exclusively by the anterior pituitary. Here we present evidence for the novel expression of FSHβ, LHβ, and the common glycoprotein-α (Cgα) in the gilthead seabream ovary. Using in situ hybridization and immunocytochemistry, FSHβ was detected in primary-growth and secondary-growth-I oocytes, LHβ was found in secondary-growth oocytes, and Cgα was observed in both primary and secondary-growth oocytes. Northern blot analyses demonstrated that Fshβ transcript is 0.6 kb in both pituitary and ovary, whereas the ovarian Lhβ transcript (1.1 kb), unexpectedly, is longer than the known pituitary Lhβ transcript (0.6 kb). Sequence analyses revealed that ovarian Lhβ is driven by a different promoter than pituitary Lhβ, which generates an additional 459 bases at the distal portion of the 5′-untranslated region of the ovarian Lhβ. Furthermore, using in vitro ovarian fragment incubation, we demonstrated that mammalian GnRH analog agonist enhanced the expression of ovarian Fshβ (up to 2.7-fold), Lhβ (up to 1.4-fold), Cgα (up to 1.8-fold), and the secretion of ovarian LH (up to 2.2-fold). In contrast, GnRH antagonist, analog E, suppressed the secretion of ovarian LH. Our findings suggest that a GnRH-gonadotropin axis is present in the gilthead seabream ovary and that FSH and LH, the well-characterized pituitary hormones, may have prominent novel roles in teleost intraovarian communication between oocytes and ovarian follicle cells.