August 21, 2014;
Hierarchical molecular events driven by oocyte-specific factors lead to rapid and extensive reprogramming.
Nuclear transfer to oocytes is an efficient way to transcriptionally reprogram somatic nuclei, but its mechanisms remain unclear. Here, we identify a sequence of molecular events that leads to rapid transcriptional reprogramming of somatic nuclei after transplantation to Xenopus oocytes. RNA-seq analyses reveal that reprogramming by oocytes results in a selective switch in transcription toward an oocyte
rather than pluripotent type, without requiring new protein synthesis. Time-course analyses at the single-nucleus
level show that transcriptional reprogramming is induced in most transplanted nuclei in a highly hierarchical manner. We demonstrate that an extensive exchange of somatic- for oocyte
-specific factors mediates reprogramming and leads to robust oocyte
RNA polymerase II binding and phosphorylation on transplanted chromatin. Moreover, genome-wide binding of oocyte
-specific linker histone B4 supports its role in transcriptional reprogramming. Thus, our study reveals the rapid, abundant, and stepwise loading of oocyte
-specific factors onto somatic chromatin as important determinants for successful reprogramming.
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Figure 1. Rapid Genome-wide Transcriptional Reprogramming in the Absence of Protein Synthesis(A) Experimental design for the RNA-seq analysis of newly transcribed mRNAs before and after NT to Xenopus oocytes. BrUTP is used to label newly transcribed RNAs.(B) Venn diagram of genes classified as activated (reprogrammed), continuously transcribed (maintained), or repressed (downregulated) after NT based on log2 count per million (log2 CPM donor cell divided by NT). False discovery rate (FDR) <0.05. Results are based on three independent experiments.(C) Representative examples of GO terms significantly enriched (FDR <0.05) in genes upregulated (reprogrammed genes in Figure 1B) and downregulated (downregulated genes in Figure 1B) after nuclear transfer.(D) Highly expressed genes in Xenopus oocytes tend to be highly expressed in transplanted nuclei. Box plots of gene expression data, with boxes demarcating the 25th–75th percentile and the median indicated by black lines. Whiskers extend to the most extreme data points with a distance to the box at most 1.5 times the box height. Expression levels are compared by reads per kilobase per million (RPKM) values. Statistical significance was calculated using t test; p <10−16.(E) Hierarchical clustering of gene expression from cultured MEFs before NT, mouse orthologs of Xenopus oocytes expressed genes, MEFs after NT to oocytes, mouse ESCs, and two-cell-stage mouse embryos.(F) Differentially expressed genes after NT in the presence or absence of CHX treatment. Genes are considered differentially expressed if log2CPM (control/CHX treatment) shows a FDR value <0.05. Results are based on three independent experiments.(G) Hierarchical clustering of gene expression from cultured cells before NT, NT oocytes, and NT oocytes with CHX treatment.See also Figure S1 and Tables S1 and S2.
Figure 2. Time-Course Analysis at the Single-Nucleus Level Reveals the Temporal Recruitment of Oocyte B4 and of Pol II Phosphorylation(A) Experimental set-up to examine binding of oocyte factors to transplanted nuclei.(B) Proportion of transplanted nuclei stained by B4, hypophosphorylated Pol IIA, Ser5P Pol II, and Ser2P Pol II at different times after NT. n = number of nuclei scored.(C and D) B4 (green), Pol IIA (red), Ser5P Pol II (blue), and Ser2P Pol II (magenta) immunofluorescence detection at the indicated times after NT of MEF nuclei. DAPI is in white. Scale bars in DAPI, 10 μm. Images represent projected Z sections (C–F).(E) Immunofluorescence detection of B4 (green) and total Pol II (red) 6 hr after nuclear transfer of MEF nuclei. Total Pol II antibody recognizes hypophosphorylated Pol IIA, Ser5P Pol II and Ser2P Pol II. Arrows indicate nuclei that are positive for B4 but negative for total Pol II. DAPI is in blue.(F) Immunofluorescence detection of B4 (green), Pol IIA (red), and Ser5P Pol II (blue). Arrowheads indicate nuclei that are positive for B4 and Pol IIA but negative for Ser5P Pol II. Arrows show nuclei positive for all markers. (G) Proportion of transplanted nuclei stained with B4, Pol IIA, Ser5P Pol II, or Ser2P Pol II, which are also stained with B4 (green), Pol IIA (orange), Ser5P Pol II (blue), or Ser2P Pol II (magenta) at 6 hr.See also Figure S2 and Table S3.
Figure 3. An Exchange from Somatic to Oocyte Transcriptional Machinery Takes Place during Reprogramming by the Xenopus Oocyte(A) Confocal imaging of YFP-RPB1 expressing nuclei (yellow) transplanted into oocytes expressing cherry-H2B (magenta). Images were recorded 15 min and 15 hr after NT. Scale bar, 20 μm. Graph at right: average mean pixel intensity per nucleus normalized to the highest value.(B) GFP-TBP nuclei (green) transplanted into oocytes expressing TBP2-cherry (red). Confocal images were recorded soon after (15 min) and 24 hr after NT. Incorporation of TBP2-cherry and disappearance of GFP-TBP are observed.(C) Experimental set-up for western blot analysis and transcriptional inhibition with aAm or Flav.(D) Transcriptional inhibition by aAm and Flav inhibits activation of Oct4 and Sox2 in NT oocytes, as examined by RT-qPCR analyses. n = 3. Data are represented as mean ± SEM.(E) Western blot analysis of total Pol II, which recognizes both Pol IIA (hypophosphorylated) and Pol IIo (hyperphosphorylated), Ser5P Pol II, Ser2P Pol II, and histone H3 in MEF nuclei 0, 6, 24, and 48 hr after NT. Arrowheads indicate Pol IIA and Pol IIo. aAm and Flav treatments of transplanted nuclei are shown.(F) Comparison of the Ser2P Pol II band intensity (red) before and after NT. The band intensity, detected by western blot, was normalized to the number of nuclei. Fold enrichment of Ser2P Pol II in NT samples over donor cells is shown in the graph. Data are represented as mean ± SEM; n = 3.See also Figure S3 and Movies S1 and S2.
Figure 4. Somatic RPB1 Is Not Sufficient for Reprogramming by the Oocyte(A) NT of aAmR RBP1 somatic cell nuclei. aAmR-RPB1 murine erythroleukemia nuclei were transplanted into oocytes in the absence or in the presence of transcriptional inhibitor aAm.(B) Quantitative analysis of Lefty (blue) and Myc (red) transcripts in oocytes transplanted with aAmR-RPB1 nuclei and cultured in the presence or in the absence of aAm for 0, 24, or 48 hr. Error bars indicate SEM. a.u., arbitrary unit.See also Figure S4.
Figure 5. Genome-wide Oocyte Linker Histone B4 Binding to the Chromatin of Transplanted Nuclei(A) The binding of B4 was analyzed in the ±20 kB region surrounding the TSS of all mouse genes. The graph shows the normalized read count for B4 (top [i]) and H3 (bottom [ii]) across this region. The longitudinal axis shows reads per billion per gene in each bin.(B) Distribution of B4 peaks in different genomic regions. B4/H3 peaks per megabase were calculated across different genomic regions. The dotted line shows the whole genome average value. Statistical significance was evaluated against peaks in the whole genome. p values were calculated using t test. ∗∗∗p <0.0001; ∗∗p < 0.001; and ∗p <0.01.(C) Same as in (A) but for the genes that are expressed (green) and not expressed (red) after NT (as determined by RNA-seq analysis in Figure 1B).(D) HHB overexpression impairs activation of reprogrammed genes that were identified by RNA-seq analysis. Gene reactivation in NT oocytes was judged by RT-qPCR analysis. Data are represented as mean ± SEM; n = 4. p values were calculated using ANOVA; ∗p <0.01.See also Figure S5.
Figure 6. Hierarchical Sequence of Events of Transcriptional Reprogramming by the Xenopus OocyteStages of nuclear reprogramming by the Xenopus oocyte. After NT to the Xenopus oocyte, somatic nuclei are bound by oocyte-specific B4. Loss of somatic Pol II and TBP starts. B4 binding is followed by the accumulation of oocyte-specific TBP2 and hypophosphorylated RPB1 (Pol II binding) derived from the oocyte, which becomes subsequently phosphorylated on serine 5. This is followed by serine 2 phosphorylation, resulting in productive transcription. The same ordered sequence of events takes place in most of transplanted nuclei within 48 hr, with slightly different kinetics.
TBP2 is a substitute for TBP in Xenopus oocyte transcription.