Prolonged FOS Activity Disrupts A Global Myogenic Transcriptional Program By Altering 3D Chromatin Architecture in Primary Muscle Progenitor Cells

Background Quiescent (non-dividing) muscle satellite cells (SCs) transition into proliferating progenitor cells that differentiate to repair skeletal muscle after injury. Recently, the AP-1 family member, FOS, was found to be rapidly and transiently induced in SCs in response to muscle damage, and its activity is required for effective SC activation and muscle repair. However, why FOS is rapidly down-regulated before SCs enter the cell cycle as progenitor cells remains unclear. In addition, whether boosting FOS expression in primary cycling muscle progenitor cells can enhance their myogenic properties needs to be evaluated. Methods In this study, we established a lentiviral, doxycycline-inducible system to explore the molecular and functional consequences of continued FOS expression in SC-derived, cycling muscle progenitor cells ex vivo. Specifically, we performed cell growth and EdU incorporation assays to measure cellular proliferation; myotube formation assays to evaluate terminal differentiation potential; and RNA-Sequencing (RNA-Seq) in conjunction with high-throughput chromosome conformation capture (Hi-C) to uncover changes in gene expression and long-range chromatin interactions. Results Persistent FOS activity in cycling muscle progenitor cells modestly enhances progenitor cell proliferation, but severely antagonizes their ability to differentiate and form myotubes. Genome-wide RNA-Seq analysis revealed that ectopic FOS activity in cycling muscle progenitor cells suppresses a global pro-myogenic gene expression program, while activating a stress-induced Mitogen-Activated Protein Kinase (MAPK) transcriptional signature. Importantly, we found various chromosomal re-organization events in A/B compartments, TADs, and long-range genomic loops near FOS-regulated differentially expressed genes in cycling muscle progenitor cells ectopically expressing FOS. Conclusions Altogether, our results suggest that elevated FOS activity in proliferating muscle progenitor cells perturbs cellular differentiation by altering the 3-dimensional (3D) chromosome organization near critical pro-myogenic genes. This work highlights the crucial importance of tightly controlling FOS expression in the muscle lineage, and raises the possibility that in states of chronic stress or disease, persistent FOS activity in muscle stem and/or progenitor cells may actually disrupt the muscle forming process. Overall design: RNA-seq and Hi-C on FOS and GFP expressing muscle satellite cells