β-catenin safeguards the ground state of pluripotency by strengthening the robustness of the transcriptional apparatus [RNA-seq]
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ABSTRACT: Mouse embryonic stem cells (ESCs) cultured with inhibitors of MEK and GSK3 (2iL) more closely resemble the pre-implantation embryo inner cell mass than cultures in serum/LIF (SL). Unveiling the differences between both systems is important for understanding development and could also assist in isolating an ESC equivalent for other mammalian species. In SL, pluripotency and cell cycle gene transcription is a multistep process requiring release of promoter-proximal paused RNA polymerase II (Pol2) by the histone acetylation reader BRD4 and the Pol2 kinase CDK9. Here, we show that recruitment of co-regulators including Mediator and Cohesin by β-catenin changes the mode of transcriptional regulation at BRD4/CDK9-bound loci in 2iL. This switch renders pluripotency genes more reliant on transcriptional initiation and less on Pol2 pause release for effective gene body elongation. Conversely, cell cycle genes are not bound by β-catenin and are still dependent on Pol2 pause release. Thus, pluripotency is more resistant to BRD4/CDK9 suppression in 2iL while self-renewal remains highly sensitive. Our findings help to explain how pluripotency is shielded in the ground state and provide insight into transcriptional adaptation upon network perturbation in other contexts.
Project description:Mouse embryonic stem cells (ESCs) cultured with inhibitors of MEK and GSK3 (2iL) more closely resemble the pre-implantation embryo inner cell mass than cultures in serum/LIF (SL). Unveiling the differences between both systems is important for understanding development and could also assist in isolating an ESC equivalent for other mammalian species. In SL, pluripotency and cell cycle gene transcription is a multistep process requiring release of promoter-proximal paused RNA polymerase II (Pol2) by the histone acetylation reader BRD4 and the Pol2 kinase CDK9. Here, we show that recruitment of co-regulators including Mediator and Cohesin by β-catenin changes the mode of transcriptional regulation at BRD4/CDK9-bound loci in 2iL. This switch renders pluripotency genes more reliant on transcriptional initiation and less on Pol2 pause release for effective gene body elongation. Conversely, cell cycle genes are not bound by β-catenin and are still dependent on Pol2 pause release. Thus, pluripotency is more resistant to BRD4/CDK9 suppression in 2iL while self-renewal remains highly sensitive. Our findings help to explain how pluripotency is shielded in the ground state and provide insight into transcriptional adaptation upon network perturbation in other contexts.
Project description:Mouse embryonic stem cells (ESCs) cultured with MEK and GSK3 inhibitors (2i) more closely resemble the inner cell mass of pre-implantation blastocysts than those cultured with serum/LIF (SL). The transcriptional mechanisms governing this pluripotent ground state are yet unresolved. Release of promoter-proximal paused RNA polymerase II (Pol2) is a multistep process necessary for pluripotency and cell cycle gene transcription in SL. Here, we show that β-catenin, stabilized by GSK3 inhibition in 2i, supplies transcriptional co-regulators including BRD4, CDK9, Mediator, Cohesin, and p300 at pluripotency loci. This selectively strengthens pluripotency loci and renders them addicted to transcription initiation for productive gene body elongation in detriment to Pol2 pause release. By contrast, cell cycle genes are not bound by β-catenin and, thus, proliferation/self-renewal are still tightly controlled by the Pol2 pause release mechanism under 2i conditions. Our findings help to explain how pluripotency is preserved in the ground state and also provide a general model for transcriptional resilience/adaptation upon network perturbation in other biological contexts.
Project description:Reactivation of the pluripotency network during somatic cell reprogramming by exogenous transcription factors involves chromatin remodeling and the recruitment of RNA polymerase II (Pol II) to target loci. Here, we report that Pol II is engaged at pluripotency promoters in reprogramming but remains paused and inefficiently released. We also show that bromodomain-containing protein 4 (BRD4) stimulates productive transcriptional elongation of pluripotency genes by dissociating the pause release factor P-TEFb from an inactive complex containing HEXIM1. Consequently, BRD4 overexpression enhances reprogramming efficiency and HEXIM1 suppresses it, whereas Brd4 and Hexim1 knockdown do the opposite. We further demonstrate that the reprogramming factor KLF4 helps recruit P-TEFb to pluripotency promoters. Our work thus provides a mechanism for explaining the reactivation of pluripotency genes in reprogramming and unveils an unanticipated role for KLF4 in transcriptional pause release. Examination of differential gene expression after overexpression of Cdk9-DN at 4 time points of somatic cell reprogramming
Project description:The BET family protein BRD4, which forms the CDK9-containing BRD4-PTEFb complex, is considered to be a master regulator of RNA polymerase II (Pol II) pause release. Because its tandem bromodomains interact with acetylated histone lysine residues, it has long been thought that BRD4 requires these bromodomains for its recruitment to chromatin and transcriptional regulatory function. Here, using rapid depletion and genetic complementation with domain deletion mutants, we demonstrate that BRD4 bromodomains are dispensable for Pol II pause release. A minimal, bromodomain-less C-terminal BRD4 fragment containing the PTEFb-interacting C-terminal motif (CTM) is instead both necessary and sufficient to mediate Pol II pause release in the absence of full-length BRD4. Although BRD4-PTEFb can associate with chromatin through acetyl recognition, our results indicate that a distinct, active BRD4-PTEFb population functions to regulate transcription independently of bromodomain-mediated chromatin association. These findings may enable more effective pharmaceutical modulation of BRD4-PTEFb activity.
Project description:Reactivation of the pluripotency network during somatic cell reprogramming by exogenous transcription factors involves chromatin remodeling and the recruitment of RNA polymerase II (Pol II) to target loci. Here, we report that Pol II is engaged at pluripotency promoters in reprogramming but remains paused and inefficiently released. We also show that bromodomain-containing protein 4 (BRD4) stimulates productive transcriptional elongation of pluripotency genes by dissociating the pause release factor P-TEFb from an inactive complex containing HEXIM1. Consequently, BRD4 overexpression enhances reprogramming efficiency and HEXIM1 suppresses it, whereas Brd4 and Hexim1 knockdown do the opposite. We further demonstrate that the reprogramming factor KLF4 helps recruit P-TEFb to pluripotency promoters. Our work thus provides a mechanism for explaining the reactivation of pluripotency genes in reprogramming and unveils an unanticipated role for KLF4 in transcriptional pause release. Refer to individual Series
Project description:Reactivation of the pluripotency network during somatic cell reprogramming by exogenous transcription factors involves chromatin remodeling and the recruitment of RNA polymerase II (Pol II) to target loci. Here, we report that Pol II is engaged at pluripotency promoters in reprogramming but remains paused and inefficiently released. We also show that bromodomain-containing protein 4 (BRD4) stimulates productive transcriptional elongation of pluripotency genes by dissociating the pause release factor P-TEFb from an inactive complex containing HEXIM1. Consequently, BRD4 overexpression enhances reprogramming efficiency and HEXIM1 suppresses it, whereas Brd4 and Hexim1 knockdown do the opposite. We further demonstrate that the reprogramming factor KLF4 helps recruit P-TEFb to pluripotency promoters. Our work thus provides a mechanism for explaining the reactivation of pluripotency genes in reprogramming and unveils an unanticipated role for KLF4 in transcriptional pause release. Pol II ChIP-seq for MEFs, ESCs and bulk populations of OSKM reprogramming intermediates at two time points.
Project description:Recent studies have found that promoter-proximal pausing occurs at most Pol2-regulated genes. DSIF and NELF function as negative elongation factors and promote Pol2 pausing. P-TEFb, whose enzymatic activity lies in the kinase Cdk9, positively regulates the transition into productive elongation by phosphorylating subunits in DSIF, NELF and Pol2. To gain insights into the interplay between these factors in regulating transcriptional pause release, we tested if knockdown of either pausing factor could bypass P-TEFb function. We find that P-TEFb function is still required for transcriptional elongation after DSIF or NELF knockdown.
Project description:Recent studies have found that promoter-proximal pausing occurs at most Pol2-regulated genes. DSIF and NELF function as negative elongation factors and promote Pol2 pausing. P-TEFb, whose enzymatic activity lies in the kinase Cdk9, positively regulates the transition into productive elongation by phosphorylating subunits in DSIF, NELF and Pol2. To gain insights into the interplay between these factors in regulating transcriptional pause release, we tested if knockdown of either pausing factor could bypass P-TEFb function. We find that P-TEFb function is still required for transcriptional elongation after DSIF or NELF knockdown. mES cells were infected with a shRNA hairpin to knockdown Spt5, Spt4, NelfE or control. The resulting cells were then treated with flavopiridol (1uM for 60 minutes). DNA was enriched by chromatin immunoprecipitation (ChIP) and analyzed by Solexa sequencing. A sample of whole cell extract was sequenced and used as the background to determine enrichment. ChIP was performed using an antibody against total RNA Pol2 (Rpb1 N-terminus, Santa Cruz sc-899).
Project description:Distal enhancers characterized by H3K4me1 mark play critical roles in developmental and transcriptional programs. However, potential roles of specific distal regulatory elements in regulating RNA Polymerase II (Pol II) promoter-proximal pause release remain poorly investigated. Here we report that a unique cohort of jumonji C domain-containing protein 6 (JMJD6) and bromodomain-containing protein 4 (Brd4) co-bound distal enhancers, termed anti-pause enhancers (A-PEs), regulate promoter-proximal pause release of a large subset of transcription units via long-range interactions. Brd4-dependent JMJD6 recruitment on A-PEs mediates erasure of H4R3me2(s), which is directly read by 7SK snRNA, and decapping/demethylation of 7SK snRNA, ensuring the dismissal of the 7SKsnRNA/HEXIM inhibitory complex. The interactions of both JMJD6 and Brd4 with the P-TEFb complex permit its activation and pause release of regulated coding genes. The functions of JMJD6/ Brd4-associated dual histone and RNA demethylase activity on anti-pause enhancers have intriguing implications for these proteins in development, homeostasis and disease. All Gro-seq(s) were designed to reveal the transcriptional targets of JMJD6 and Brd4, and assess the role of JMJD6 and Brd4 in Pol II promoter-proximal pause release. All ChIP-seq(s) were designed to understand the unique features, associated molecular mechanisms and functions of the anti-pause enhancers (A-PEs) discovered in the current study.
Project description:Reactivation of the pluripotency network during somatic cell reprogramming by exogenous transcription factors involves chromatin remodeling and the recruitment of RNA polymerase II (Pol II) to target loci. Here, we report that Pol II is engaged at pluripotency promoters in reprogramming but remains paused and inefficiently released. We also show that bromodomain-containing protein 4 (BRD4) stimulates productive transcriptional elongation of pluripotency genes by dissociating the pause release factor P-TEFb from an inactive complex containing HEXIM1. Consequently, BRD4 overexpression enhances reprogramming efficiency and HEXIM1 suppresses it, whereas Brd4 and Hexim1 knockdown do the opposite. We further demonstrate that the reprogramming factor KLF4 helps recruit P-TEFb to pluripotency promoters. Our work thus provides a mechanism for explaining the reactivation of pluripotency genes in reprogramming and unveils an unanticipated role for KLF4 in transcriptional pause release.