[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Germ cells divide and differentiate in a unique local microenvironment under the control of somatic cells. Signals released in this niche instruct oocyte reentry into the meiotic cell cycle. Once initiated, the progression through meiosis and the associated programme of maternal messenger RNA translation are thought to be cell autonomous. Here we show that translation of a subset of maternal mRNAs critical for embryo development is under the control of somatic cell inputs. Translation of specific maternal transcripts increases in oocytes cultured in association with somatic cells and is sensitive to EGF-like growth factors that act only on the somatic compartment. In mice deficient in amphiregulin, decreased fecundity and oocyte developmental competence is associated with defective translation of a subset of maternal mRNAs. These somatic cell signals that affect translation require activation of the PI(3)K-AKT-mTOR pathway. Thus, mRNA translation depends on somatic cell cues that are essential to reprogramme the oocyte for embryo development.
Full-text · Article · Nov 2013 · Nature Cell Biology
[Show abstract][Hide abstract] ABSTRACT: Developmental regulatory genes have both activating (H3K4me3) and repressive (H3K27me3) histone modifications in embryonic stem cells (ESCs). This bivalent configuration is thought to maintain lineage commitment programs in a poised state. However, establishing physiological relevance has been complicated by the high number of cells required for chromatin immunoprecipitation (ChIP). We developed a low-cell-number chromatin immunoprecipitation (low-cell ChIP) protocol to investigate the chromatin of mouse primordial germ cells (PGCs). Genome-wide analysis of embryonic day 11.5 (E11.5) PGCs revealed H3K4me3/H3K27me3 bivalent domains highly enriched at developmental regulatory genes in a manner remarkably similar to ESCs. Developmental regulators remain bivalent and transcriptionally silent through the initiation of sexual differentiation at E13.5. We also identified >2,500 "orphan" bivalent domains that are distal to known genes and expressed in a tissue-specific manner but silent in PGCs. Our results demonstrate the existence of bivalent domains in the germline and raise the possibility that the somatic program is continuously maintained as bivalent, potentially imparting transgenerational epigenetic inheritance.
[Show abstract][Hide abstract] ABSTRACT: Enhancers and antisense RNAs play key roles in transcriptional regulation through differing mechanisms. Recent studies have demonstrated that enhancers are often associated with non-coding RNAs (ncRNAs), yet the functional role of these enhancer:ncRNA associations is unclear. Using RNA-Sequencing to interrogate the transcriptomes of undifferentiated mouse embryonic stem cells (mESCs) and their derived neural precursor cells (NPs), we identified two novel enhancer-associated antisense transcripts that appear to control isoform-specific expression of their overlapping protein-coding genes. In each case, an enhancer internal to a protein-coding gene drives an antisense RNA in mESCs but not in NPs. Expression of the antisense RNA is correlated with expression of a shorter isoform of the associated sense gene that is not present when the antisense RNA is not expressed. We demonstrate that expression of the antisense transcripts as well as expression of the short sense isoforms correlates with enhancer activity at these two loci. Further, overexpression and knockdown experiments suggest the antisense transcripts regulate expression of their associated sense genes via cis-acting mechanisms. Interestingly, the protein-coding genes involved in these two examples, Zmynd8 and Brd1, share many functional domains, yet their antisense ncRNAs show no homology to each other and are not present in non-murine mammalian lineages, such as the primate lineage. The lack of homology in the antisense ncRNAs indicates they have evolved independently of each other and suggests that this mode of lineage-specific transcriptional regulation may be more widespread in other cell types and organisms. Our findings present a new view of enhancer action wherein enhancers may direct isoform-specific expression of genes through ncRNA intermediates.
[Show abstract][Hide abstract] ABSTRACT: Classical approaches to determine structures of noncoding RNA (ncRNA) probed only one RNA at a time with enzymes and chemicals, using gel electrophoresis to identify reactive positions. To accelerate RNA structure inference, we developed fragmentation sequencing (FragSeq), a high-throughput RNA structure probing method that uses high-throughput RNA sequencing of fragments generated by digestion with nuclease P1, which specifically cleaves single-stranded nucleic acids. In experiments probing the entire mouse nuclear transcriptome, we accurately and simultaneously mapped single-stranded RNA regions in multiple ncRNAs with known structure. We probed in two cell types to verify reproducibility. We also identified and experimentally validated structured regions in ncRNAs with, to our knowledge, no previously reported probing data.
[Show abstract][Hide abstract] ABSTRACT: The developmental and evolutionary mechanisms behind the emergence of human-specific brain features remain largely unknown. However, the recent ability to compare our genome to that of our closest relative, the chimpanzee, provides new avenues to link genetic and phenotypic changes in the evolution of the human brain. We devised a ranking of regions in the human genome that show significant evolutionary acceleration. Here we report that the most dramatic of these 'human accelerated regions', HAR1, is part of a novel RNA gene (HAR1F) that is expressed specifically in Cajal-Retzius neurons in the developing human neocortex from 7 to 19 gestational weeks, a crucial period for cortical neuron specification and migration. HAR1F is co-expressed with reelin, a product of Cajal-Retzius neurons that is of fundamental importance in specifying the six-layer structure of the human cortex. HAR1 and the other human accelerated regions provide new candidates in the search for uniquely human biology.