Cloning, characterisation, and expression of three oestrogen receptors (ERα, ERβ1 and ERβ2) in the European sea bass, Dicentrarchus labrax

Abstract

Three oestrogen receptor [ER] subtypes have been described in teleost fish, namely ERα, and two ERβ subtypes, called ERβ1 and ERβ2 (or ERβ and ERγ in Atlantic croaker). Their expression during embryonic development and gonadal growth has evoked interest in their potential role in sexual differentiation and gonadal development in fish. We cloned three oestrogen receptors from adult liver (sb-ERα cDNA) and ovary (partial sb-ERβ1 and sb-ERβ2 cDNAs) of the European sea bass, and according to their phylogenetic relatedness to other ERs in teleosts, named them sea bass [sb-] ERα, ERβ1 and ERβ2. Deduced amino acid numbers for sb-ERα, sb-ERβ1 and sb-ERβ2 were 639, 517 and 608, respectively, representing in the case of sb-ERβ1 and sb-ERβ2 about 90% of the open reading frame. Highest amino acid identities were found for sb-ERα with eelpout ERα (88.7%), for sb-ERβ1 with Atlantic croaker ERγ (85.8%), and for sb-ERβ2 with Atlantic croaker ERβ (90.1%). Southern analysis confirmed that all three sea bass oestrogen receptors (sb-ERs) are the products of three distinct genes. In adult sea bass, ERα was predominantly expressed in liver and pituitary, while sb-ERβ1 and sb-ERβ2 were more ubiquitously expressed, with highest expression levels in pituitary. In a mixed-sex population of juvenile sea bass, sb-ERα expression was significantly elevated in gonads at 200 days posthatch (dph), while for sb-ERβ1 and sb-ERβ2 highest expression levels were observed in gonads at 250 dph. For sb-ERβ2, expression was also significantly higher in the brain at 250 dph. The cloning of these three ER subtypes in the European sea bass together with the results obtained on expression levels in adult and juvenile animals has given us the foundation to investigate their possible role in sexual differentiation and development in this species in future studies.