Project description:Conversion of fibroblasts into skeletal muscle cells by overexpression of the transcription factor MyoD has been widely employed over past decades in a multitude of experimental settings. However, the potential of additional myogenic regulatory factors such as Myf5, Myf6 and Myog to elicit a myogenic conversion has been significantly less studied. To address this disparity, we set out to investigate the role of MRFs in fibroblast conversion into skeletal muscle stem and differentiated cells. Surprisingly, we found that Myf6, a transcription factor associated with postnatal late-stage myogenesis, efficiently converted fibroblasts into myotubes alone, or alternatively into expandable Pax7-expressing induced myogenic progenitor cells (iMPCs) in the presence of defined small molecules. To investigate reprogramming dynamics in a tightly regulated system, we generated an inducible Myf6 overexpression transgenic mouse model, which enabled dissection of transcription and epigenetic dynamics unique to iMPC formation. Mechanistically, we characterized the chromatin binding of Myf6, as well as its protein binding partners, during iMPC reprogramming and conventional transdifferentiation, revealing shared and different targets and binding partners. To further investigate its role, we surprisingly found that endogenous Myf6 is dispensable for iMPC formation by MyoD and small molecules. In accordance with this observation, we demonstrated that subjecting MyoD-KO fibroblasts to reprogramming with Myf6 and small molecules gave rise to expandable MyoD-KO iMPCs, indicating that MyoD is similarly dispensable for iMPC formation and self-renewal. Multi-omics analyses of Myf6-derived MyoD-KO iMPCs uncovered upregulated genes that compensated for the lack of MyoD, whose downregulation during iMPC production mitigated Pax7 expression, highlighting their roles as reprogramming regulators. Collectively, these results illustrate a redundancy between MRFs’ function in myogenic cellular conversions, highlighting an unexpected potency for Myf6 in the induction of a muscle stem cell fate in vitro.

Project description: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.

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Dissecting the Role of Myf6 in Facilitating Fibroblast Conversion into Myogenic Progenitor and Differentiated Cells [RNA-Seq]

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