Molecular characterization and expression pattern of zona pellucida proteins in gilthead seabream (Sparus aurata)

The developing oocyte is surrounded by an acellular envelope that is composed of 2-4 isoforms of zona pellucida (ZP) proteins. The ZP proteins comprise the ZP1, ZP2, ZP3, and ZPX isoforms. While ZP1 (ZPB) and ZP3 (ZPC) are present in all species, ZP2 (ZPA) is not found in teleost fish and ZPX is not found in mammals. In the present study, we identify and characterize the ZP1, ZP3 and ZPX isoforms of gilthead seabream. Furthermore, by analyzing the conserved domains, which include the external hydrophobic patch and the internal hydrophobic patch, we show that ZP2 and ZPX are closely related isoforms. ZP proteins are synthesized in either the liver or ovary of most teleosts. Only in rainbow trout has it been shown that zp3 has dual transcription sites. In gilthead seabream, all four mRNA isoforms are transcribed in both the liver and ovary, with zp1a, zp1b, and zp3 being highly expressed in the liver, and zpx being primarily expressed in the ovary. However, determination of the ZP proteins in plasma showed high levels of ZP1b, ZP3, and ZPX, with low or non-detectable levels of ZP1a. In similarity to other teleost ZPs, the hepatic transcription of all four ZP isoforms is under estrogenic control. Previously, we have shown that cortisol can potentiate estrogen-induced ZP synthesis in salmonids, and now we show that this is not the case in the gilthead seabream. The present study shows for the first time the endocrine regulation of a teleost ZPX isoform, and demonstrates the dual-organ transcriptional activities of all the ZP proteins in one species.