MiR-26 regulates ddx3x expression in medaka (Oryzias latipes) gonads

MicroRNAs (miRNAs) participate in the reproductive process by regulating the expression of target genes, however, there are few studies on the regulation of mRNA target by miRNAs in medaka (Oryzias latipes). Our previous data showed that miR-202-5p has a sex-biased expression and miR-26 is abundantly expressed in both ovary and testis of medaka. In this study, DEAD-box helicase 3, X-linked (ddx3x) was proposed to be a putative target of miR-26 and miR-202-5p through bioinformatic analysis. Here, medaka ddx3x (Olddx3x) was cloned and sequence alignment analysis indicated that Olddx3x is highly conserved and has more than 90% identity with some other teleosts. Reverse transcription polymerase chain reaction (RT-PCR) showed that Olddx3x was highly expressed in testis and ovary. Meanwhile, chromogenic in situ hybridization manifested that Olddx3x was abundantly expressed in the early oocytes in the ovary and in various stages of spermatogenesis, especially the obvious signal in the testis of spermatocytes and sperm. Dual-luciferase reporter assay and the injection of miR-26 agomir and antagomir demonstrated that the expression of Olddx3x was regulated by miR-26 but not miR-202-5p. The overall results revealed that miR-26 may perform a vital role in medaka reproduction development by regulating the expression of ddx3x.