Deletion of ddx4 ovary-specific transcript causes downregulation of sycp1 and derepression of DNA transposons in zebrafish ovaries
Ddx4 (DEAD-box helicase 4) is a well-established marker gene for early-stage germlines and is required for the development of germ cells. Alternative splicing of ddx4 produces a long transcript that is exclusively expressed in zebrafish ovaries. The function of this ovary-specific transcript (ddx4-L) remains unclear. In this study, using the ddx4-L knockout zebrafish model, we found that elimination of ddx4-L results in a decrease in fertilization rate and in the number of mature eggs laid by the female mutants. Transcriptome analysis was performed to identify the underlying changes of gene expression between WT and ddx4-L knockout ovaries. A total of 1134 differentially expressed genes were identified, of which 524 were upregulated and 610 were downregulated. Functional enrichment analysis showed that the terms of negative regulation of fertilization, sperm-egg recognition process, and oocyte meiosis might be affected by elimination of ddx4-L. Furthermore, we found that Sycp1, a synaptonemal complex protein involved in meiosis and fertility, was dramatically decreased in the ddx4-L knockout ovaries at both mRNA and protein levels. Moreover, the activities of transposable elements were analyzed based on the RNA-seq data. The result showed that elimination of ddx4-L causes the derepression of DNA transposons, a subclass of transposable elements, in zebrafish ovaries. In conclusion, the aforementioned findings indicate that the ovary-specific transcript of ddx4 plays an important role in oocyte development and egg quality, possibly by maintaining sycp1 expression and repressing DNA transposons. Our work provide novel insights into the functions and regulatory mechanisms of ddx4 in zebrafish oogenesis. Overall design: Ovaries were dissected from three WT and three ddx4-E4? zebrafish at 6 mpf, respectively. Six total RNA samples were extracted and subjected to RNA sequencing. Differentially expressed genes (DEGs) and transposable elements (TEs) between WT and ddx4-E4? zebrafish ovaries were analyzed.