The pituitary gonadotropins (GtHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are key regulators of gametogenesis in teleosts. However, little is known about the physiological mechanisms by which GtHs regulate asynchronous oocyte development in multiple-spawning marine fishes. We cloned cDNAs encoding GtH receptors (FSHR and LHR) from chub mackerel (Scomber japonicus). FSH and LH were purified by anion-exchange chromatography, gel filtration, and concanavalinA- agarose. When expressed in mammalian cells, FSHR and LHR responded strongly to their own ligands. By separating LH into two subunits by the use of reverse-phase chromatography, we found that the beta-subunit is responsible for signal transduction and the alpha-subunit may be important for holding hormone-receptorcomplex. In situ hybridization showed that only fshr was expressed in prefollicle and granulosa cellsin oocytes at the perinucleolus and cortical alveolus stages, suggesting that FSH is involved in the primary and early secondary growth of oocytes. In ovarian follicles during vitellogenesis, both fshr and lhr were expressed in granulosa and thecal cells, and lhr was strongly expressed during germinal vesicle migration (GVM). Real-time PCR analysis of stagedependent fshr and lhr expression showed thatfshr expression was high in ovarian follicles throughout vitellogenesis and decreased during GVM, whereas lhr expression was low in early vitellogenesis, but increased markedly in the late phase of vitellogenesis, remaining high during GVM. These findings suggest that switching of the expression of FSHR to LHR controls the effects of FSH and/or LH on vitellogenesis and final oocyte maturation via steroid production in granulosa and thecal cells.
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