The pluripotent mammalian epiblast undergoes unusually fast cell proliferation. This rapid growth is expected to generate a high transcriptional demand, but the underlying mechanisms remain unknown. We show here that the chromatin remodeler Chd1 is required for transcriptional output and development of the mouse epiblast. Chd1−/− embryos exhibit proliferation defects and increased apoptosis, are smaller than controls by E5.5 and fail to grow, to become patterned or to gastrulate. Removal of p53 allows progression of Chd1−/− mutants only to E7.0-8.0, highlighting the crucial requirement for Chd1 during early post-implantation development. Chd1−/− embryonic stem cells (ESCs) have a self-renewal defect and a genome-wide reduction in transcriptional output at both known mRNAs and intergenic transcripts. These transcriptional defects were only uncovered when cell number-normalized approaches were used, and correlate with a lower engagement of RNAP II with transcribed genes in Chd1−/− ESCs. We further show that Chd1 directly binds to ribosomal DNA, and that both Chd1−/− epiblast cells in vivo and ESCs in vitro express significantly lower levels of ribosomal RNA. In agreement with these findings, mutant cells in vivo and in vitro exhibit smaller and more elongated nucleoli. Thus, the RNA output by both Pol I and II is reduced in Chd1−/− cells. Our data indicate that Chd1 promotes a globally elevated transcriptional output required to sustain the distinctly rapid growth of the mouse epiblast.
Author contributions
M.R.-S. directed the project. M.G.-A. performed all of the experiments in embryos, with technical assistance from P.W. M.G.-A. also analyzed heterochromatin levels and nucleolar structure in ESCs. M.S. carried out analyses of self-renewal, differentiation, transcriptional output and S2p RNAP in ESCs. He also developed the Chd1-Flag ESCs, with assistance from R.N. F.M.K. carried out gene targeting, Southern blotting and cell cycle analyses in ESCs. C.O. analyzed the RNA-seq data with aid and supervision from J.S.S. and M.R.-S. A.B.-K. performed Chd1-Flag and total RNAP II ChIP. C.-J.L. and M.S. derived and genotyped ESCs. P.W., F.M.K. and M.G.-A. managed the mouse colony. P.W. contributed to the culture of ESCs under the supervision of M.G.-A. and M.S. M.G.-A., M.S. and M.R.-S. designed experiments, interpreted the results and wrote the manuscript. M.G.-A. led the revisions of the manuscript.
Funding
M.G.-A. was partially supported by a T32 grant from the National Institutes of Health (NIH) to the UCSF Center for Reproductive Sciences. F.M.K. was supported by the Agency for Science, Technology and Research (Singapore). R.N. was supported by the SFSU CIRM bridges program. This work was supported by NIH grant U54HD055764, by a Sontag Foundation Distinguished Scientist Award to J.S.S. and by NIH New Innovator Award DP2OD004698 to M.R.-S. Deposited in PMC for release after 12 months.
