Differential involvement of the three nuclear estrogen receptors during ovarian development in European sea bass (Dicentrarchus labrax)

Abstract

Estrogens are involved in a wide range of processes in vertebrate reproduction, acting through ligand-activation of their specific cognate receptors. Nuclear estrogen receptors (Esr) are transcription factors that bind to estrogen response elements on gene promoters to regulate their expression. In most teleosts, three nuclear estrogen receptor subtypes have been identified (Esr1, Esr2a and Esr2b), probably involved in different physiological functions. Indeed, differences in ligand binding affinity and seasonal expression patterns in reproductive tissues among Esr subtypes, suggest distinct roles during oogenesis, vitellogenesis and testicular development. In European sea bass, the complete cDNA sequence of esr1 and partial cDNA sequences of esr2a and esr2b had already been described, together with the brain distribution of their mRNAs and the transcriptional activity of Esr1. However, no information was available on the transcriptional activity of Esr2a and Esr2b, or on the significance of the Esr subtypes in other reproductive tissues. This study is focused on investigating the role of the Esr subtypes in European sea bass ovarian development and their endocrine regulation. We isolated the complete cDNAs of sea bass esr2a and esr2b and functionally characterized them using HEK293 cells. The coding genes of the three Esr subtypes are highly expressed in reproductive related tissues such as pituitary, gonad and liver. Quantification of esr1, esr2a and esr2b expression in the ovary during a whole reproductive cycle showed different expression patterns depending on stage and subtype, suggesting differential roles of the three receptors in the regulation of oogenesis. Finally, for the first time in teleosts, localization of the different estrogen receptor subtypes in ovarian follicles and regulation by gonadotropins are described. Results showed differences among subtypes depending on the gonadal stage, reinforcing the hypothesis that each estrogen receptor has a specific role on ovarian development.