RNA‐methyltransferase Nsun5 controls the maternal‐to‐zygotic transition by regulating maternal mRNA stability

BACKGROUND: RNA modification‐induced ovarian dysgenesis appears to be necessary for ovary development. However, how m(5)C (5‐methylcytosine)‐coordinating modificatory transcripts are dynamically regulated during oogenesis, and ovarian development is unknown. The purpose of this study was to determine whether NOP2/Sun RNA methyltransferase 5 (Nsun5) deletion leads to suppression of ovarian function and arrest of embryonic development. The regulation of mRNA decay and stability by m(5)C modification is essential at multiple stages during the maternal‐to‐zygotic (MZT) transition. METHODS: Mouse ovaries and oocytes with Nsun5 (KO) and the KGN cell line were subjected to m(5)C identification, alternative splicing analysis and protein expression. BS‐m(5)C‐seq, real‐time polymerase chain reaction, Western blot, immunofluorescence and actinomycin D treatment assays were used. In particular, BS‐m(5)C‐seq revealed a dynamic pattern of m(5)C sites and genes in the ovaries between Nsun5 (KO) and WT mice at the 2‐month and 6‐month stages. Diverse bioinformatic tools were employed to identify target genes for Nsun5. RESULTS: Here, a maternal mRNA stability study showed that deletion of the m(5)C methyltransferase Nsun5 obstructs follicular development and ovarian function, which leads directly to inhibition of embryogenesis and embryo development. Dynamic analysis of m(5)C revealed that the level of m(5)C decreased in a time‐dependent manner after Nsun5 knockout. Regarding the molecular mechanism, we found that Nsun5 deficiency caused a m(5)C decline in the exon and 3′UTR regions that influenced the translation efficiency of Mitotic arrest deficient 2 like 2 (MAD2L2) and Growth differentiation factor 9 (GDF9) in the ovary. Mechanistic investigation of alternative splicing indicated that Nsun5 (KO) triggers aberrant events in the exon region of Brd8. CONCLUSIONS: Nsun5 loss arrests follicular genesis and development in ovarian aging, indicating that Nsun5/m(5)C‐regulated maternal mRNA stabilization is essential for MZT transition.