Integrative molecular roadmap for reprogramming mouse fibroblasts into induced myogenic stem and progenitor cells [bulk RNA-seq]

Direct lineage reprogramming provides a unique system to study cell fate transitions and unearth molecular mechanisms that safeguard cellular identity. We previously reported on direct conversion of mouse fibroblasts into induced myogenic progenitor cells (iMPCs) by means of transient MyoD overexpression in concert with small molecules treatment. Here we employed integrative multi-omic assays to delineate the molecular landscape of fibroblast reprogramming into iMPCs in comparison to transdifferentiation into myogenic cells solely by MyoD overexpression. Utilizing bulk RNA-sequencing and mass spectrometry we characterized molecular regulators and pathways that endow a myogenic stem cell identity on fibroblasts only in the presence of small molecule treatment. We further demonstrate that iMPC reprogramming is a stepwise process, commencing with the appearance of myofibers and committed myogenic progenitors prior to the formation of satellite cell-like myogenic progenitors that express a suite of stem cell markers including Pax7, Dek, Myf5, Sox8, and Dmrt2. To directly compare iMPCs to satellite cell-derived primary myoblasts, we employed a fluorescent Pax7-GFP reporter to purify Pax7+ cells from the heterogenous iMPC cultures and assess their equivalency to FACS-purified Pax7-GFP+ myoblasts. We demonstrate that Pax7+ iMPCs share molecular attributes with myoblasts, however also express genes and signaling pathways that are indicative of activated satellite cells including Carm1, Dlk1, Lgr5, Fos, Dek, Calcr and Pitx3. We further establish that iMPC formation and maintenance is dependent on the Notch pathway, as small molecule inhibition of Notch abrogates iMPC formation and derails stable iMPC cultures via depletion of Pax7 expressing cells. Lastly, using single cell RNA-sequencing we determine the cell populations that comprise iMPCs in addition to reconstructing the differentiation trajectory present in these heterogenous cultures. We demonstrate that a highly proliferative activated satellite cell-like population differentiates into committed myoblasts and myocytes that further give rise to myofibers that express mature muscle markers, thus capturing a dynamic differentiation program in vitro. Collectively, this study charts a molecular blueprint for reprogramming fibroblasts into myogenic stem and progenitor cells and further establishes the fidelity of stable iMPC cultures in capturing skeletal muscle regeneration in vitro for disease modeling and basic research applications. We established a dox-inducible overexpression system with the muscle transcription factor MyoD in transgenic MEFs carrying a Pax7-nGFP reporter. We reprogrammed these MEFs into multinucleated myotubes by Dox (MyoD) treatment or into induced myogenic progenitor cells (iMPCs) by Dox+F/R/C (MyoD+F/R/C) treatment. Utilizing this system, we conducted bulk RNA-seq transcriptome profiling of MEFs subjected to MyoD overexpression alone or with small molecules (F/R/C) every two days. In addition, we performed RNA-Seq analysis of FACS-purified Pax-nGFP+ iMPCs and Pax-nGFP+ myoblasts. Lastly, we also analyzed via RNA-Seq bulk myoblasts and established dox-independent iMPC clones.