在哺乳动物中,17-β羟基类固醇脱氢酶2(Hsd17b2)酶特异性催化C17羟基的氧化,并有效调节雌激素和雄激素的活性,以预防由激素紊乱引起的疾病。然而,hsd17b2基因在动物性别分化中的功能尚不清楚。稻田鳗鱼(黄翅目),一种具有小基因组大小(2n=24)的雌雄同体的雌雄同体鱼,通常用作研究脊椎动物性别分化机制的理想模型。因此,在这项研究中,克隆了鳗鱼hsd17b2基因cDNA,并测定了其mRNA表达谱。克隆的hsd17b2cDNA片段为1230bp,包括一个1107bp的开放阅读框,编码368个氨基酸残基,具有保守的催化亚基。此外,实时定量逆转录聚合酶链反应(RT-qPCR)分析表明,hsd17b2mRNA在发育早期的卵巢中表达强烈,在肝脏和肠道中较弱,几乎不在睾丸和其他组织中。特别是,早期发现hsd17b2mRNA在幼鱼和卵睾丸的卵巢中表达达到峰值,并最终在性腺中从晚期卵睾丸下降到睾丸。同样,化学原位杂交结果表明,在I-II期卵细胞和卵母细胞的细胞质中主要检测到hsd17b2mRNA信号,随后在Ⅲ-Ⅳ期集中在卵母细胞周围的颗粒细胞中,但在成熟卵母细胞和雄性生殖细胞中检测不到。有趣的是,在稻田鳗鱼卵巢里,17β-雌二醇(E2)或他莫昔芬(17β-雌二醇抑制剂,E2I)在低浓度(10ng/mL)下诱导,而在高浓度(100ng/mL)下通过E2I诱导增加。另一方面,褪黑激素(MT)和氟他胺(雄激素抑制剂,AI)诱导可显著降低鳗鱼卵巢hsd17b2mRNA的表达。本研究为揭示鱼类性别分化的机制提供了线索。我们的研究结果表明,hsd17b2基因可能是稻田鳗鱼和其他雌雄同体鱼类的性分化和性别逆转的关键调节因子。
In mammals, 17-beta hydroxysteroid dehydrogenase 2 (Hsd17b2) enzyme specifically catalyzes the oxidation of the C17 hydroxyl group and efficiently regulates the activities of estrogens and androgens to prevent diseases induced by hormone disorders. However, the functions of the hsd17b2 gene involved in animal sex differentiation are still largely unclear. The ricefield eel (Monopterus albus), a protogynous hermaphroditic fish with a small genome size (2n = 24), is usually used as an ideal model to study the mechanism of sex differentiation in vertebrates. Therefore, in this study, hsd17b2 gene cDNA was cloned and its mRNA expression profiles were determined in the ricefield eel. The cloned cDNA fragment of hsd17b2 was 1230 bp, including an open reading frame of 1107 bp, encoding 368 amino acid residues with conserved catalytic subunits. Moreover, real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR) analysis showed that hsd17b2 mRNA expressed strongly in the ovaries at early developmental stages, weakly in liver and intestine, and barely in testis and other tissues. In particular, hsd17b2 mRNA expression was found to peak in ovaries of young fish and ovotestis at the early stage, and eventually declined in gonads from the late ovotestis to testis. Likewise, chemical in situ hybridization results indicated that the hsd17b2 mRNA signals were primarily detected in the cytoplasm of oogonia and oocytes at stage I-II, subsequently concentrated in the granulosa cells around the oocytes at stage Ⅲ-Ⅳ, but undetectable in mature oocytes and male germ cells. Intriguingly, in ricefield eel ovaries, hsd17b2 mRNA expression could be significantly reduced by 17β-estradiol (E2) or tamoxifen (17β-estradiol inhibitor, E2I) induction at a low concentration (10 ng/mL) and increased by E2I induction at a high concentration (100 ng/mL). On the other hand, both the melatonin (MT) and flutamide (androgen inhibitor, AI) induction could significantly decrease hsd17b2 mRNA expression in the ovary of ricefield eel. This study provides a clue for demonstrating the mechanism of sexual differentiation in fish. The findings of our study imply that the hsd17b2 gene could be a key regulator in sexual differentiation and modulate sex reversal in the ricefield eel and other hermaphroditic fishes.