Hierarchical and stage-specific regulation of murine cardiomyocyte maturation by serum response factor

After birth, cardiomyocytes (CM) acquire numerous adaptations in order to efficiently pump blood throughout an animal’s lifespan. How this maturation process is regulated and coordinated is poorly understood. Here, we perform a CRISPR/Cas9 screen in mice and identify serum response factor (SRF) as a key regulator of CM maturation. Mosaic SRF depletion in neonatal CMs disrupts many aspects of their maturation, including sarcomere expansion, mitochondrial biogenesis, transverse-tubule formation, and cellular hypertrophy. Maintenance of maturity in adult CMs is less dependent on SRF. This stage-specific activity is associated with developmentally regulated SRF chromatin occupancy and transcriptional regulation. SRF directly activates genes that regulate sarcomere assembly and mitochondrial dynamics. Perturbation of sarcomere assembly but not mitochondrial dynamics recapitulates SRF knockout phenotypes. SRF overexpression also perturbs CM maturation. Together, these data indicate that carefully balanced SRF activity is essential to promote CM maturation through a hierarchy of cellular processes orchestrated by sarcomere assembly

Hierarchical and stage-specific regulation of murine cardiomyocyte maturation by serum response factor

After birth, cardiomyocytes (CM) acquire numerous adaptations in order to efficiently pump blood throughout an animal’s lifespan. How this maturation process is regulated and coordinated is poorly understood. Here, we perform a CRISPR/Cas9 screen in mice and identify serum response factor (SRF) as a key regulator of CM maturation. Mosaic SRF depletion in neonatal CMs disrupts many aspects of their maturation, including sarcomere expansion, mitochondrial biogenesis, transverse-tubule formation, and cellular hypertrophy. Maintenance of maturity in adult CMs is less dependent on SRF. This stage-specific activity is associated with developmentally regulated SRF chromatin occupancy and transcriptional regulation. SRF directly activates genes that regulate sarcomere assembly and mitochondrial dynamics. Perturbation of sarcomere assembly but not mitochondrial dynamics recapitulates SRF knockout phenotypes. SRF overexpression also perturbs CM maturation. Together, these data indicate that carefully balanced SRF activity is essential to promote CM maturation through a hierarchy of cellular processes orchestrated by sarcomere assembly

Mapping cell type-specific transcriptional enhancers using high affinity, lineage-specific Ep300 bioChIP-seq

Understanding the mechanisms that regulate cell type-specific transcriptional programs requires developing a lexicon of their genomic regulatory elements. We developed a lineage-selective method to map transcriptional enhancers, regulatory genomic regions that activate transcription, in mice. Since most tissue-specific enhancers are bound by the transcriptional co-activator Ep300, we used Cre-directed, lineage-specific Ep300 biotinylation and pulldown on immobilized streptavidin followed by next generation sequencing of co-precipitated DNA to identify lineage-specific enhancers. By driving this system with lineage-specific Cre transgenes, we mapped enhancers active in embryonic endothelial cells/blood or skeletal muscle. Analysis of these enhancers identified new transcription factor heterodimer motifs that likely regulate transcription in these lineages. Furthermore, we identified candidate enhancers that regulate adult heart- or lung- specific endothelial cell specialization. Our strategy for tissue-specific protein biotinylation opens new avenues for studying lineage-specific protein-DNA and protein-protein interactions. DOI: http://dx.doi.org/10.7554/eLife.22039.00

Interrogating translational efficiency and lineage-specific transcriptomes using ribosome affinity purification

Transcriptional profiling is a useful strategy to study development and disease. Approaches to isolate RNA from specific cell types, or from specific cellular compartments, would extend the power of this strategy. Previous work has shown that isolation of genetically tagged ribosomes (translating ribosome affinity purification; TRAP) is an effective means to isolate ribosome-bound RNA selectively from transgene-expressing cells. However, widespread application of this technology has been limited by available transgenic mouse lines. Here we characterize a TRAP allele (Rosa26[superscript fsTRAP]) that makes this approach more widely accessible. We show that endothelium-specific activation of Rosa26[superscript fsTRAP] identifies endothelial cell-enriched transcripts, and that cardiomyocyte-restricted TRAP is a useful means to identify genes that are differentially expressed in cardiomyocytes in a disease model. Furthermore, we show that TRAP is an effective means for studying translational regulation, and that several nuclear-encoded mitochondrial genes are under strong translational control. Our analysis of ribosome-bound transcripts also shows that a subset of long intergenic noncoding RNAs are weakly ribosome-bound, but that the majority of noncoding RNAs, including most long intergenic noncoding RNAs, are ribosome-bound to the same extent as coding transcripts. Together, these data show that the TRAP strategy and the Rosa26[superscript fsTRAP] allele will be useful tools to probe cell type-specific transcriptomes, study translational regulation, and probe ribosome binding of noncoding RNAs.American Heart AssociationNational Heart, Lung, and Blood Institute (U01HL098166)National Heart, Lung, and Blood Institute (U01HL098188)National Heart, Lung, and Blood Institute (R01HL095712

VEGF amplifies transcription through ETS1 acetylation to enable angiogenesis

Release of promoter-proximally paused RNA polymerase II (RNAPII) is a recently recognized transcriptional regulatory checkpoint. The biological roles of RNAPII pause release and the mechanisms by which extracellular signals control it are incompletely understood. Here we show that VEGF stimulates RNAPII pause release by stimulating acetylation of ETS1, a master endothelial cell transcriptional regulator. In endothelial cells, ETS1 binds transcribed gene promoters and stimulates their expression by broadly increasing RNAPII pause release. 34 VEGF enhances ETS1 chromatin occupancy and increases ETS1 acetylation, enhancing its binding to BRD4, which recruits the pause release machinery and increases RNAPII pause release. Endothelial cell angiogenic responses in vitro and in vivo require ETS1-mediated transduction of VEGF signaling to release paused RNAPII. Our results define an angiogenic pathway in which VEGF enhances ETS1–BRD4 interaction to broadly promote RNAPII pause release and drive angiogenesis