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

RNA modification-induced ovarian dysgenesis appears to be necessary for ovary development. However, how m⁵ 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 m⁵ C modification is essential at multiple stages during the maternal-to-zygotic (MZT) transition.

Mouse ovaries and oocytes with Nsun5^KO and the KGN cell line were subjected to m⁵ C identification, alternative splicing analysis and protein expression. BS-m⁵ C-seq, real-time polymerase chain reaction, Western blot, immunofluorescence and actinomycin D treatment assays were used. In particular, BS-m⁵ C-seq revealed a dynamic pattern of m⁵ 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.

Here, a maternal mRNA stability study showed that deletion of the m⁵ C methyltransferase Nsun5 obstructs follicular development and ovarian function, which leads directly to inhibition of embryogenesis and embryo development. Dynamic analysis of m⁵ C revealed that the level of m⁵ C decreased in a time-dependent manner after Nsun5 knockout. Regarding the molecular mechanism, we found that Nsun5 deficiency caused a m⁵ 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.

Nsun5 loss arrests follicular genesis and development in ovarian aging, indicating that Nsun5/m⁵ C-regulated maternal mRNA stabilization is essential for MZT transition.