Project description:The mechanisms by which Pax7 promotes skeletal muscle stem (satellite) cell identity are not yet understood. We have taken advantage of pluripotent stem cells wherein the induced expression of Pax7 robustly initiates the muscle program and enables the generation of muscle precursors that repopulate the satellite cell compartment upon transplantation. Pax7 binding was excluded from H3K27 tri-methylated regions, suggesting that recruitment of this factor is circumscribed by chromatin state. Further, Pax7 binding provoked localized chromatin remodeling, including the acquisition of enhancer-associated histone marks and induction of chromatin accessibility. Conversely, removal of Pax7 led to rapid reversal of these features on a subset of enhancers. Another cohort of Pax7 binding sites exhibited a durably accessible and remodeled chromatin state after Pax7 removal, and persistent enhancer accessibility was associated with subsequent binding by muscle regulatory factors. Our studies provide new insights into Pax7 and the epigenetic landscape of skeletal muscle stem cells.
Project description:Adult muscle stem cells, also known as muscle satellite cells, which are the resident tissue stem cells of skeletal muscle, provide myonuclei for postnatal muscle growth and for maintenance and regeneration in adults. Satellite cells specifically express the transcription factor pax7. The purpose of this study was to identify pax7 target genes and clarify the role of pax7 in muscle stem cell maintenance. We succeeded in generating Pax7-YFP knockin mice (Pax7-YFP KI) that can visualize endogenous pax7 expressed in satellite cells with YFP fluorescent protein. Novel pax7 target genes were identified by ChIP-sequencing (chromatin immunoprecipitation) analysis with muscle stem cells of Pax7-YFP KI mice.
Project description:SILAC based protein correlation profiling using size exclusion of protein complexes derived from Mus musculus tissues (Heart, Liver, Lung, Kidney, Skeletal Muscle, Thymus)
Project description:SILAC based protein correlation profiling using size exclusion of protein complexes derived from seven Mus musculus tissues (Heart, Brain, Liver, Lung, Kidney, Skeletal Muscle, Thymus)
Project description:Using Hi-C, promoter-capture Hi-C (pCHi-C), and other genome-wide approaches in skeletal muscle progenitors that inducibly express a master transcription factor, Pax7, we systematically characterized at high-resolution the spatio-temporal re-organization of compartments and promoter-anchored interactions as a consequence of myogenic commitment and differentiation. To further investigate transcriptional regulatory mechanisms governed by Pax7, we used immuno-affinity purification and mass spectrometric sequencing to identify the compendium of Pax7-associated proteins in muscle progenitors. Although Pax7-associated proteins have been identified in myoblasts, it is likely that such an approach would fail to uncover progenitor
specific interactions with this protein. We therefore isolated chromatin from iPax7 cells
engineered with a single, inducible copy of the Flag-tagged Pax7 transgene integrated next to the Hprt locus, which is expected to maintain expression levels comparable
to those of satellite cells in the presence of Dox. Through purification of Flag-Pax7, we
identified a cohort of factors and complexes involved in gene activation or repression and remodeling of chromatin and genome architecture that were substantially enriched compared to the uninduced control. We also identified a large cohort of sequence-specific TFs, including several that were shown to play an essential role in muscle stem cells (e.g., Foxk1, Six1, Runx1, Tead1, Nfix) and that are recruited to Pax7-bound enhancers in muscle cells. Importantly, we also confirmed multiple interactions from our proteomic screen through immunoprecipitation and western blotting.
Project description:Using Hi-C, promoter-capture Hi-C (pCHi-C), and other genome-wide approaches in inducible Pax7-expressing skeletal muscle progenitors that inducibly express a master transcription factor, Pax7, we systematically characterized at high-resolution the spatio-temporal re-organization of compartments and promoter-anchored interactions as a consequence of myogenic commitment and differentiation. We identified key promoter-enhancer interaction motifs, namely, cliques and networks, and interactions that were dependent on Pax7 binding. Remarkably, we found that the majority of super-enhancers were bound by Pax7 and a cadre of associated factors that maintained an epigenetic memory of active enhancers in the absence of Pax7. Lastly, we identified a previously uncharacterized Pax7-bound enhancer hub that simultaneously regulates the essential myosin heavy chain cluster during skeletal muscle cell differentiation. Our studies lay the groundwork for understanding the three-dimensional conformation of chromatin in muscle stem cells.
Project description:Skeletal muscle harbors quiescent stem cells termed satellite cells and proliferative progenitors termed myoblasts, which play pivotal roles during muscle regeneration. However, current technology does not allow permanent capture of these cell populations in vitro. Here, we show that ectopic expression of the myogenic transcription factor MyoD, combined with exposure to small molecules, reprograms mouse fibroblasts into expandable induced myogenic progenitor cells (iMPCs). iMPCs express key skeletal muscle stem and progenitor cell markers including Pax7 and Myf5 and give rise to Dystrophin-expressing myofibers upon transplantation, a subset of which maintain Pax7 expression in vivo and sustain serial regenerative responses. Similar to satellite cells, iMPCs originate from Pax7+ cells and require Pax7 itself for maintenance. Finally, we show that iMPCs can be established from muscle tissue following small molecule exposure alone. This study thus reports on a robust approach to derive expandable myogenic stem/progenitor-like cells from multiple differentiated cell types.
Project description:Skeletal muscle harbors quiescent stem cells termed satellite cells and proliferative progenitors termed myoblasts, which play pivotal roles during muscle regeneration. However, current technology does not allow permanent capture of these cell populations in vitro. Here, we show that ectopic expression of the myogenic transcription factor MyoD, combined with exposure to small molecules, reprograms mouse fibroblasts into expandable induced myogenic progenitor cells (iMPCs). iMPCs express key skeletal muscle stem and progenitor cell markers including Pax7 and Myf5 and give rise to Dystrophin-expressing myofibers upon transplantation, a subset of which maintain Pax7 expression in vivo and sustain serial regenerative responses. Similar to satellite cells, iMPCs originate from Pax7+ cells and require Pax7 itself for maintenance. Finally, we show that iMPCs can be established from muscle tissue following small molecule exposure alone. This study thus reports on a robust approach to derive expandable myogenic stem/progenitor-like cells from multiple differentiated cell types.
Project description:Derivation of human skeletal muscle in vitro with human pluripotent stem cells (hPSCs) opens new avenues for deciphering essential, but poorly understood aspects of transcriptional regulation in myogenic specification and relevance to rare genetic diseases. We characterized the transcriptional landscape of distinct human myogenic stages, including OCT4::EGFP+ PSCs, MSGN1::EGFP+ presomites, PAX7::EGFP+ skeletal muscle progenitors, MYOG::EGFP+ myoblasts, and multinucleated myotubes. We defined signature gene expression profiles from each population with unbiased clustering analysis, which provided unique insights into the transcriptional dynamics of human myogenesis from undifferentiated hPSCs to fully differentiated myotubes. Using knock-out strategy, we identified TWIST1 as a critical factor in maintenance of human PAX7::EGFP+ putative skeletal muscle progenitors, providing an explanation for the musculoskeletal symptoms of a rare genetic disease, Saethre-Chotzen syndrome. We have established a foundation for future studies to identify regulators of human myogenic ontogeny.