Up-regulation of YBX1 during MZT in goat, bovine, mice, and human
During goat early embryo development, YBX1 was gradually up-regulated from oocyte to blastocyst (Fig. 1A); YBX2 was up-regulated from the 2-cell embryos to the morula, while YBX3 was down-regulated at the 8-cell stage compared to the 4- and 16-cell stage embryos. During bovine early embryo development, the expression of YBX1 was significantly increased in the 8-cell embryos compared to the 4-cell embryos, whereas YBX2 was down-regulated during MZT (Fig. 1B). In mice, the expression of YBX1 was increased after fertilization and further increased at the 4-cell embryos compared to the 2-cell embryo. YBX2 and YBX3 was down-regulated during ZGA in mice (Fig. 1C). The expression of YBX1 in human were similar with that of mice (Fig. 1D). Conservative analysis revealed high homology of the protein sequence among goat, bovine, mice, and human (Supplementary Figure S1). These data suggest conserved expression pattern of YBX1 in mammalian embryos and that YBX1 might play vital roles during MZT.
Knockdown of YBX1 impeded the early embryo development
To confirm its role in preimplantation, we knocked down YBX1 by siRNA microinjection (Fig. 2A). The expression of YBX1 was successfully inhibited (Fig. 2B), while the expression of YBX2 and YBX3 showed no statistical change in YBX1 knockdown embryos at the 8-cell stage (Fig. 2C). Specifically, the percentage of blastocyst was significantly decreased in YBX1 knockdown embryos compared to the controls (12.96 ± 1.51% vs. 41.39 ± 1.20%, p < 0.01; Fig. 2D-E). Moreover, embryos that blocked at the 2- and 4-cell stage were increased in YBX1 knockdown embryos compared to the controls (Fig. 2F), suggesting essential roles of YBX1 during early embryo development.
Knockdown of YBX1 compromised the transcriptional activity during ZGA
To determine whether gene expression profiles correlated with the treatment, we analyzed RNA-Seq data by unsupervised hierarchical clustering. Embryos that clustered together were at the same group (Fig. 3A), indicating good quality of the RNA-seq data. Using DEseq2, we obtained 5154 differentially expressed genes (DEGs). Of which, 3531 genes were down-regulated, while 1623 genes were up-regulated in YBX1 knockdown embryos compared to the controls (Fig. 3B-D, Supplementary Table S1), suggesting compromised the transcriptional activity during ZGA by YBX1 knockdown. We further performed 5-EU staining to confirm the notion. As expected, 5-EU was weakly stained in the 4-cell embryos, and markedly increased in the 8-cell embryos. However, in the 8-cell stage YBX1 knockdown embryos, the level of 5-EU was decreased compared to the 4- and 8-cell embryos in the control group (p < 0.01, p < 0.001; Fig. 3E-F). In mice, ZGA initiates at the 2-cell embryos. The 5-EU was strongly stained at the 2-cell embryos, while the signal intensity of 5-EU was significantly decreased at 2-cell YBX1 knockdown embryos in mice (p < 0.001; Fig. 3G-H). These data suggest that knockdown of YBX1 impaired transcriptional activity during ZGA in both goat and mice.
It is important to know the pathway that genes were de-repressed and/or downregulated in knockdown experiments. As shown in Fig. 4A, the 1623 up-regulated genes were enriched in chromosomal region, DNA replication, meiotic cell cycle, ERBB signaling pathway, DNA geometric change, and DNA duplex unwinding as revealed by GO annotation. The 3531 down-regulated genes were enriched in RNA/mRNA/ncRNA metabolic process, ncRNA processing, regulation of mRNA metabolic process, methylation, RNA modification, RNA/mRNA splicing, and regulation of RNA/mRNA stability (Fig. 4B). KEGG analysis revealed that the down-regulated genes were enriched in RNA transport, spliceosome, and ribosome (Fig. 4C). To further confirm GO and KEGG annotation results, we performed GSEA analysis. As expected, the DEGs were enriched in ncRNA processing (p.adjust < 0.001), RNA methylation (p.adjust < 0.05), RNA methyltransferase activity (p.adjust < 0.05), RNA processing (p.adjust < 0.001), and spliceosome (p.adjust < 0.05; Fig. 3D-E). These data suggest that YBX1 regulate MZT by regulation of RNA splicing and RNA stability.
YBX1 regulates alternative splicing during MZT
We further analyzed the AS events in the 8-cell stage YBX1 knockdown embryos using the RNA-seq data. As shown in Fig. 5A, 18001 skipped exon (SE), 195 retained intron (RI), 2768 mutually exclusive exons (MXE), 154 alternative 5`splice site (A5SS), and 230 alternative 3`splice site (A3SS) was identified at the 8-cell embryos. With the △PSI > 0.05 and the FDR < 0.05, we identified 3284 differential AS events. SE (76.52%), followed by MXE (21.29%) appears to be the most abundant differential AS events (Fig. 5B, Supplementary Table S2). For example, knockdown of YBX1 promoted the sixth exon skipping in Breast Cancer Type 1 (BRCA1, Fig. 5C), but inhibited the ninth exon skipping in Eukaryotic Translation Initiation Factor 3 Subunit I (EIF3I, Fig. 5D), the eleven-exon skipping in Embryonic Ectoderm Development (EED, Fig. 5E), and the sixth exon skipping in Heterogeneous Nuclear Ribonucleoprotein M (HNRNPM, Fig. 5F).
Transcriptional profile of splicing factors and genes of spliceosome pathway was characterized at the 8-cell stage YBX1 knockdown embryos. During ZGA, Serine and Arginine Rich Splicing Factor (SRSF)1/2/3/10, Splicing Factor 3b Subunit (SF3B) 1/3/5, HNRNPM, HNRNPK, HNRNPU, DExD-Box Helicase 39B (DDX39B) showed no statistically changed. However, they were down-regulated in the 8-cell stage YBX1 knockdown embryos compared to the controls (Fig. 5G). In addition, these genes were not only highly correlated with each other, but also predicted to be targeted with the YBX1 (Fig. 5H). Taken together, our data suggest that YBX1 was in regulation of AS during the MZT process.
Knockdown of YBX1 impairs maternal mRNA decay during MZT
To confirm YBX1 is associated with RNA stability, we established a highly correlated hub genes network. As shown in Fig. 5A, the N6-methyladenosine-related genes (YTHDF2/3, METTL3, and IGF2BP1), Proteasome (Prosome, Macropain) 26S Subunit (PSM) family, Heterogeneous Nuclear Ribonucleoprotein (HNRNP), Decapping mRNA 1A (DCP1A), and DCP2 were highly correlated. Moreover, EIF4G1, HNRNPD, HNRNPM, HNRNPR, HNRNPU, and IGF2BP1 were predicted to target with YBX1 directly (Fig. 6A). The expression of ZFP36 (p < 0.01), YTHDF2 (p < 0.05), IGF2BP1 (p < 0.01), EIF4G1 (p < 0.01), HNRNPM (p < 0.01), HNRNPU (p < 0.01), DCP1A (p < 0.05), DCP2 (p < 0.001), and METTL3 (p < 0.05) were significantly decreased in YBX1 knockdown embryos at the 8-cell stage compared to the controls (Fig. 6B), suggesting that YBX1 might regulate mRNA decay during MZT.
We further analyzed the expression of maternal mRNAs in YBX1 knockdown embryos. There were 1322 M-decay genes expressed in both the knockdown and the NC group. Of which, 295 maternal genes were up-regulated, while 317 maternal genes were down-regulated in YBX1 knockdown embryos at the 8-cell stage compared to the controls (Fig. 6C). As for the Z-decay genes, 904 genes were detected in the RNA-seq data. In the 8-cell stage YBX1 knockdown embryos, the expression of 184 Z-decay genes was increased, while the expression of 244 Z-decay genes was decreased compared to the controls (Fig. 5D). Taken together, these data demonstrated that knockdown of YBX1 impaired the maternal mRNA decay.