Identification of the cyclooxygenase (COX) gene and its role in ovarian development and ovulation of the oriental river prawn Macrobrachium nipponense

Abstract

Cyclooxygenase (COX) is the rate-limiting enzyme in the synthesis of prostaglandins (PGs), which are involved in ovarian development and ovulation in crustaceans. In this study, complementary DNA (cDNA) of COX in Macrobrachium nipponense (MnCOX) was identified, and its function in ovarian development and ovulation was investigated by quantitative real-time PCR (qPCR), fluorescence in situ hybridization (FISH), RNA interference (RNAi) and arachidonic acid (AA), dexamethasone (DEX) and DEX+ prostaglandin E2 (PGE2) administration. The results showed that the complete cDNA of MnCOX is 3238 bp, including an 1848 bp open reading frame (ORF) that encodes 615 amino acids. The MnCOX protein conservatively possesses a membrane-binding domain, an epidermal growth factor-like (EGF-like) domain, an animal haem peroxide domain, three N-glycosylation sites and seven essential catalytic residues. The richest MnCOX transcript of nine tissues was found in the gill, followed by the ovary and intestine. In six ovarian development stages, the MnCOX transcript was the richest in the stage Ⅴ ovary. FISH showed that the MnCOX transcript was present in the cytoplasm of oocytes. MnCOX knockdown led to decreases in PGE2 content, expression levels of the vitellogenin (Vg) and vitellogenin receptor (Vgr) genes and the gonadsomatic index (GSI) of prawns. Moreover, oocyte development was delayed, and the oviposition rate and the number of eggs ovulated were reduced after MnCOX knockdown. AA administration increased the expression of MnCOX, PGE2 content and the oviposition rate of prawns. However, DEX administration decreased the expression of MnCOX, PGE2 content, oviposition rate, and the number of eggs ovulated by prawns. The addition of PGE2 to DEX injection rescued the negative effect of DEX on the ovulation of M. nipponense. This investigation illustrates that MnCOX plays an important role in regulating ovarian development and ovulation in M. nipponense by affecting the synthesis of PGs.