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Ovarian Carbonyl Reductase-Like 20ß-Hydroxysteroid Dehydrogenase Shows Distinct Surge in Messenger RNA Expression During Natural and Gonadotropin-Induced Meiotic Maturation in Nile Tilapia¹

^a Laboratory of Reproductive Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan ^b Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan ^c Inland Station, National Research Institute of Aquaculture, Tamaki, Mie 519-0423, Japan ^d CREST, Japan Science Technology Corporation, Kawaguchi, Saitama 332-0012, Japan

Meiotic maturation in fish is accomplished by maturation-inducing hormones. 17,20ß-Dihydroxy-4-pregnen-3-one (17,20ß-DP) was identified as the maturation-inducing hormone of several teleosts, including Nile tilapia. A cDNA encoding 20ß-hydroxysteroid dehydrogenase (20ß-HSD), the enzyme that converts 17-hydroxyprogesterone to 17,20ß-DP, was cloned from the ovarian follicle of Nile tilapia. Genomic Southern analysis indicated that 20ß-HSD probably exists as a single copy in the genome. The Escherichia coli-expressed cDNA product oxidized both carbonyl and steroid compounds, including progestogens, in the presence of NADPH. Carbonyl reductase-like 20ß-HSD is broadly expressed in various tissues of tilapia, including ovary, testis, and gill. Northern blot and reverse transcription polymerase chain reaction analyses during the 14-day spawning cycle revealed that the expression of 20ß-HSD in ovarian follicles is low from Day 0 to Day 8 after spawning and is not detectable on Day 11. Distinct expression was evident at Day 14, the day of spawning. In males, 20ß-HSD expression was observed continually in mature testes but not in immature testes of 30-day-old fish. In vitro incubation of postvitellogenic immature follicles (corresponding to Day 11 after spawning) with hCG induced the expression of 20ß-HSD mRNA transcripts within 1–2 h, followed by the final meiotic maturation of oocytes. In tissues such as gill, muscle, brain, and pituitary, however, hCG treatment did not induce any changes in the levels of mRNA transcripts. Actinomycin D blockade of hCG-induced 20ß-HSD expression and final oocyte maturation demonstrated the involvement of transcriptional factors. The carbonyl reductase-like 20ß-HSD plays an important role in the meiotic maturation of tilapia gametes.

¹ This work was supported in part by Grants-in-Aids for Research for the Future (JSPS-RFTF 96L00401) to Y.N. from the Japan Society for the Promotion of Science, for Scientific Research from the Japanese Ministry of Education, Science, Sports, and Culture, Japan, and CREST of JST (Japan Science and Technology). This work was also supported by fellowships from the Japan Society for the Promotion of Science for Young Scientists to B.S., C-C.S., and X-T.C.

² Correspondence: Yoshitaka Nagahama, Laboratory of Reproductive Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan. FAX: 81 564 55 7556; nagahama{at}nibb.ac.jp

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