Characterization and Functional Analysis of the 17-Beta Hydroxysteroid Dehydrogenase 2 (hsd17b2) Gene during Sex Reversal in the Ricefield Eel (Monopterus albus)

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 <i>hsd17b2</i> gene involved in animal sex differentiation are still largely unclear. The ricefield eel (<i>Monopterus albus</i>), a protogynous hermaphroditic fish with a small genome size (2<i>n</i> = 24), is usually used as an ideal model to study the mechanism of sex differentiation in vertebrates. Therefore, in this study, <i>hsd17b2</i> gene cDNA was cloned and its mRNA expression profiles were determined in the ricefield eel. The cloned cDNA fragment of <i>hsd17b2</i> 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 <i>hsd17b2</i> mRNA expressed strongly in the ovaries at early developmental stages, weakly in liver and intestine, and barely in testis and other tissues. In particular, <i>hsd17b2</i> 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 <i>hsd17b2</i> 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, <i>hsd17b2</i> 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 <i>hsd17b2</i> 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 <i>hsd17b2</i> gene could be a key regulator in sexual differentiation and modulate sex reversal in the ricefield eel and other hermaphroditic fishes.