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 HRAS ^G12V 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 HRAS ^G12V levels. While intermediate HRAS ^G12V levels increase the rate of conversion, high levels of HRAS ^G12V induce senescence. Through chemogenetic tuning, we set optimal MAPK activity for high rates of conversion in the absence of HRAS mutants. 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.