Chemogenetic tuning reveals optimal MAPK signaling for cell-fate programming

Cell states evolve through the combined activity of signaling pathways and gene regulatory networks. While transcription factors can direct cell fate, these factors rely on a cell state that is receptive to transitions in cell identity. How signaling levels contribute to the emergence of receptive cell states remains poorly defined in primary cells. Using a well-defined model of direct conversion, we examined how levels of the MAPK-activating oncogene HRASG12V influence direct conversion of primary fibroblasts to induced motor neurons. We demonstrate that an optimal ‘Goldilocks’ level of MAPK signaling efficiently drives cell-fate programming. Rates of direct conversion respond biphasically to increasing HRASG12V levels. While intermediate HRASG12V levels increase the rate of conversion, high levels of HRASG12V induce senescence. Through chemogenetic tuning, we set optimal MAPK activity for high rates of conversion in the absence of HRAS mutants. In addition to proliferation, MAPK signaling influences conversion by regulating Ngn2 activity, offering two distinct mechanisms of MAPK-mediated control in direct conversion. As MAPK pathways influence cell-fate transitions in development and disease, our results highlight the need to tune therapeutic interventions within a non-monotonic landscape that is shaped by genetics and levels of gene expression. Transcriptional profiling of cells during the process of direct conversion of primary mouse embryonic fibroblasts (MEFs) to induced motor neurons (iMNs). MEFs were infected with NIL, NIL + SDD, NIL + SDDIR, NIL + RISDD (NIL is Ngn2-2A-Isl1-2A-Lhx3; SDD is SNAP-p53DD, SDDIR is SDD-IRES-HRAS(G12V); RISDD is HRAS(G12V)-IRES-SDD). Samples were collected at 4 days post infection (dpi) were sorted by fluorescence-activated cell sorting (FACS) for live, single cells for conditions with only NIL, and on SNAP+ for cells expressing SDD.