Molecular characterization and expression profile of the estrogen receptor α gene during different reproductive phases in Monopterus albus

To understand the molecular mechanism of estrogen and to evaluate the role of the estrogen receptor in mediating estrogen action, the full-length cDNA of estrogen receptor α (ERα) was cloned from Monopterus albus, and its expression pattern and distribution were investigated. The ERα cDNA of M. albus includes an open reading frame of 1863 bp, a 140-bp 5'-untranslated region and a 797-bp 3'-untranslated region. Amino acid sequence homology analysis showed that the Monopterus albus ERα has a moderate degree of similarity with Sebastes schlegelii, Zoarces viviparus and Haplochromis burtoni (81.1%, 80.7% and 80.4%, respectively). Quantitative PCR results showed that the highest level of ERα expression was in the liver; the next highest level of expression was observed in the gonads, where it was expressed at high levels particularly in the ovary in developmental stages IV and V and in the testis in developmental stage II/III. Immunohistochemistry analysis showed that ERα was present as slender particles distributed mainly in the membranes of spermatocytes and oocytes in the testis and ovary, whereas no positive signal was observed in the cytoplasm of sperm cells. This report describes the first molecular characterization of full-length ERα and its tissue-specific distribution in M. albus.