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. 2017 Sep 21;67(6):1059-1067.e4.
doi: 10.1016/j.molcel.2017.08.003. Epub 2017 Aug 31.

The RNA m6A Reader YTHDF2 Is Essential for the Post-transcriptional Regulation of the Maternal Transcriptome and Oocyte Competence

Ivayla Ivanova  1 , Christian Much  1 , Monica Di Giacomo  2 , Chiara Azzi  2 , Marcos Morgan  1 , Pedro N Moreira  1 , Jack Monahan  3 , Claudia Carrieri  1 , Anton J Enright  4 , Dónal O'Carroll  5
Affiliations

The RNA m6A Reader YTHDF2 Is Essential for the Post-transcriptional Regulation of the Maternal Transcriptome and Oocyte Competence

Ivayla Ivanova et al. Mol Cell. .

Abstract

YTHDF2 binds and destabilizes N6-methyladenosine (m6A)-modified mRNA. The extent to which this branch of m6A RNA-regulatory pathway functions in vivo and contributes to mammalian development remains unknown. Here we find that YTHDF2 deficiency is partially permissive in mice and results in female-specific infertility. Using conditional mutagenesis, we demonstrate that YTHDF2 is autonomously required within the germline to produce MII oocytes that are competent to sustain early zygotic development. Oocyte maturation is associated with a wave of maternal RNA degradation, and the resulting relative changes to the MII transcriptome are integral to oocyte quality. The loss of YTHDF2 results in the failure to regulate transcript dosage of a cohort of genes during oocyte maturation, with enrichment observed for the YTHDF2-binding consensus and evidence of m6A in these upregulated genes. In summary, the m6A-reader YTHDF2 is an intrinsic determinant of mammalian oocyte competence and early zygotic development.

Keywords: YTHDF2; m(6)A; m(6)A-reader; maternal transcriptome; oocyte maturation.

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Figures

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Graphical abstract
Figure 1
Figure 1
The RNA m6A Reader YTHDF2 Is Expressed in Multiple Tissues, in Mouse Fibroblasts, and in the Germline (A) Schematic representation of the YTHDF2 protein, the Ythdf2HA-Fl allele, and the HA-YTHDF2 fusion protein. (B) Western blot using anti-HA and anti-α-tubulin antibodies on the indicated tissue and cell line lysates from Ythdf2HA-Fl/HA-Fl and wild-type (WT). (C) Top panel is a schematic representation of spermatogenesis. Below are shown confocal immunofluorescent images of testis sections stained with anti-HA antibody (green) and γ-H2AX (red) of the indicated spermatogenic cells from Ythdf2+/+ and Ythdf2HA-Fl/HA-Fl mice. Scale bar, 10 μm. (D) Schematic representation of folliculogenesis and oocyte maturation. Abbreviations: GV, germinal vesicle oocyte; MII, metaphase II-arrested oocyte. (E) Confocal immunofluorescent images of Ythdf2+/+ and Ythdf2HA-Fl/HA-Fl ovary sections stained with anti-HA antibody (green) and Hoechst (blue) for primordial, primary, secondary, early antral, and antral follicular stages. Scale bar, 30 μm. Top right corner square is a magnified image of the oocyte in the respective follicle. (F) Confocal immunofluorescence images of GV and MII oocytes of wild-type and homozygous Ythdf2HA-Fl mice stained with anti-HA antibody (green) and Hoechst (blue) are shown as indicated. Scale bar, 20 μm. See also Figure S1.
Figure 2
Figure 2
YTHDF2 Is Required for Female Fertility (A) Western blot using anti-HA and anti-α-tubulin antibodies on testis lysates from Ythdf2HA-Fl/+ and Ythdf2−/−. (B) Table of the numbers and percentages of pups at weaning and the expected Mendelian numbers of animals per genotype from Ythdf2+/− intercrosses from mixed and C57Bl6 genetic background. (C) The number of pups born per plug from Ythdf2+/− and Ythdf2−/− male mice is shown. The number (n) of animals tested, the mean and SD are indicated (t test; n.s., p > 0.05). (D) Representative PAS stained testis section from wild-type and Ythdf2−/− mice. Scale bar, 50 μm. (E) The number of pups born per plug from Ythdf2+/− and Ythdf2−/− female mice is shown. The number (n) of animals tested, the mean, and SD are indicated (t test; ∗∗∗∗p < 0.00001). (F) Representative PAS-stained ovary section from wild-type and Ythdf2−/− mice. CL indicates corpus luteum. Scale bar, 100 μm. See also Figure S1.
Figure 3
Figure 3
YTHDF2 Is Maternally Required for Oocyte Maturation and Early Zygotic Development (A) Confocal immunofluorescence with anti-HA antibody (green) and Hoechst (blue) on ovary sections from homozygous Ythdf2HA-Fl and Ythdf2mCKO mice are shown. Primordial (Pr) and secondary (S) follicle stage are indicated. Scale bar, 30 μm. (B) Number of pups born per plug from Ythdf2CTL and Ythdf2mCKO female mice. The number (n) of animals, mean, and SD are indicated (t test;∗∗∗p < 0.0001). (C) PAS-stained ovary sections from Ythdf2CTL and Ythdf2mCKO females. CL indicates corpus luteum. Scale bar, 100 μm. (D) Number of MII oocytes isolated from oviduct of hormone-primed Ythdf2CTL and Ythdf2mCKO female mice. The number (n) of animals, the mean, and SD are indicated (t test; n.s., p > 0.05). (E) Percentage of normal MII oocytes isolated from hormone-primed Ythdf2CTL and Ythdf2mCKO females (n indicates number of oocytes). The morphology of MII oocytes was assessed through immunofluorescent staining with anti-β-tubulin antibody (green) and Hoechst (blue). Representative images for Ythdf2CTL and Ythdf2mCKO MII oocytes are shown, with the zona pellucida indicated by a white dashed circle. Scale bar, 20 μm. (F) Percentage of degenerated, sperm entry, and two pronuclei zygotes isolated from hormone-primed and stud male-mated Ythdf2CTL and Ythdf2mCKO female mice at embryonic day 0.5. The number (n) of zygotes assessed is indicated. Confocal immunofluorescent images of representative Ythdf2CTL and Ythdf2mCKO zygotes stained with Hoechst (blue) are shown, with the zona pellucida indicated by a white dashed circle. The female and male pronuclei are indicated with female and male signs, respectively; Pb denotes polar bodies. Scale bar, 20 μm. (G) Percentage of degenerated, abnormal two-cell, two-cell, three- to four-cell, and six- to eight-cell zygotes isolated from stud male-mated hormone-primed Ythdf2CTL and Ythdf2mCKO female mice at embryonic day 2.5. The number of zygotes assessed is indicated. Confocal immunofluorescent images of representative Ythdf2CTL 8-cell zygote (Panel I) and Ythdf2mCKO arrested abnormal two-cell zygotes (Panels II–IV) stained with anti-β-tubulin (green) and Hoechst (blue) are shown. Scale bar, 20 μm.
Figure 4
Figure 4
YTHDF2 Regulates Maternal Transcript Dosage during Oocyte Maturation (A) Schematic representation of oocyte maturation with a graph indicating the approximate levels of maternal RNA and transcription. Abbreviations: GV, germinal vesicle oocyte; GVBD, germinal vesicle break down oocyte; MI, metaphase I oocyte; and MII, metaphase II-arrested oocyte. (B) Expression scatterplot showing the relative average expression of transcripts from GV and MII oocytes. In red are highlighted the genes that are significantly changed (p ≤ 0.05) with a fold difference greater or equal to 2 between GV and MII oocytes. Analysis was done on biological three to four replicas. (C) Expression scatterplot showing the relative average expression of transcripts between Ythdf2CTL and Ythdf2mCKO MII oocytes. Significantly deregulated (p < 0.05) genes with a fold change greater than or equal to 2 are shown in red. Analysis was done on biological triplicates. (D) Gene ontology analysis for the upregulated genes in Ythdf2mCKO MII oocytes; the top ten most significant processes identified are shown. (E) Expression scatterplot showing the relationship between transcriptome changes during oocyte maturation (shown on the y axis) and changes in Ythdf2mCKO with respect to Ythdf2CTL MII oocytes (shown on the x axis). The significantly upregulated and downregulated genes in Ythdf2mCKO MII oocytes are shown in red and blue points, respectively. The unchanged transcripts in oocyte maturation and in Ythdf2CTL versus Ythdf2mCKO MII oocytes are indicated by horizontal and vertical dashed red lines, respectively. The number of genes in the respective gates is indicated. (F) Graph representation of Loess smoothed sum of YTHDF2-binding motif GACU/A occurrences ±400 nt around the stop codon (left panel) and around the start codon (right panel). Data are shown for the top 1,000 most-upregulated transcripts (red), 1,000 downregulated transcripts (blue), and 1,000 transcripts whose expression remained unchanged (green) in Ythdf2mCKO MII oocytes. Only enrichment of the upregulated genes compared to unchanged genes around the stop codon is statistically significant (p < 0.05). (G) Prevalence of m6A peaks from public mouse datasets around the gene bodies of upregulated, downregulated, and unchanged transcripts in Ythdf2mCKO MII oocytes. Gene bodies are scaled to the same length in each case. Only enrichment of the upregulated genes compared to unchanged genes around the stop codon is statistically significant (p < 0.05). See also Figures S2, S3, and S4.
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