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Transcription factor-driven cell fate engineering in pluripotency induction, transdifferentiation, and forward reprogramming requires efficiency, speed, and maturity for widespread adoption and clinical translation. Here, we used Oct4, Sox2, Klf4, and c-Myc driven pluripotency reprogramming to evaluate methods for enhancing and tailoring cell fate transitions, through directed evolution with iterative screening of pooled mutant libraries and phenotypic selection. We identified an artificially evolved and enhanced POU factor (ePOU) that substantially outperforms wild-type Oct4 in terms of reprogramming speed and efficiency. In contrast to Oct4, not only can ePOU induce pluripotency with Sox2 alone, but it can also do so in the absence of Sox2 in a three-factor ePOU/Klf4/c-Myc cocktail. Biochemical assays combined with genome-wide analyses showed that ePOU possesses a new preference to dimerize on palindromic DNA elements. Yet, the moderate capacity of Oct4 to function as a pioneer factor, its preference to bind octamer DNA and its capability to dimerize with Sox2 and Sox17 proteins remain unchanged in ePOU. Compared with Oct4, ePOU is thermodynamically stabilized and persists longer in reprogramming cells. In consequence, ePOU: 1) differentially activates several genes hitherto not implicated in reprogramming, 2) reveals an unappreciated role of thyrotropin-releasing hormone signaling, and 3) binds a distinct class of retrotransposons. Collectively, these features enable ePOU to accelerate the establishment of the pluripotency network. This demonstrates that the phenotypic selection of novel factor variants from mammalian cells with desired properties is key to advancing cell fate conversions with artificially evolved biomolecules.

The functional consequences of cancer-associated missense mutations are unclear for the majority of proteins. We have previously demonstrated that the activity of SOX and Pit-Oct-Unc (POU) family factors during pluripotency reprogramming can be switched and enhanced with rationally placed point mutations. Here, we interrogated cancer mutation databases and identified recurrently mutated positions at critical structural interfaces of the DNA-binding domains of paralogous SOX and POU family transcription factors. Using the conversion of mouse embryonic fibroblasts to induced pluripotent stem cells as functional readout, we identified several gain-of-function mutations that enhance pluripotency reprogramming by SOX2 and OCT4. Wild-type SOX17 cannot support reprogramming but the recurrent missense mutation SOX17-V118M is capable of inducing pluripotency. Furthermore, SOX17-V118M promotes oncogenic transformation, enhances thermostability and elevates cellular protein levels of SOX17. We conclude that the mutational profile of SOX and POU family factors in cancer can guide the design of high-performance reprogramming factors. Furthermore, we propose cellular reprogramming as a suitable assay to study the functional impact of cancer-associated mutations.

Significance This work investigated the in vivo role of Paxbp1, a poorly studied nuclear protein, in regulating adult mouse muscle stem cells (MuSCs). By deleting Paxbp1 in adult mouse quiescent MuSCs, we found that Paxbp1 -null MuSCs were unable to reenter the cell cycle to proliferate upon muscle injury and subsequently underwent apoptosis, resulting in a total failure in injury-induced muscle regeneration. Mechanistically, we found that Paxbp1 controls a late cell-growth checkpoint by targeting mTORC1. Loss of Paxbp1 in MuSCs led to increased levels of reactive oxygen species that in turn triggered p53 activation and induction of multiple p53 target genes, some of which contributed to cell-cycle arrest (e.g., Cdkn1a ), apoptosis (e.g., Apaf1 ), and impaired mTORC1 signaling (e.g., Sesn2 and Ddit4 ).

Summary Directed biomolecular evolution is widely used to tailor and enhance enzymes, fluorescent proteins, and antibodies but has hitherto not been applied in the reprogramming of mammalian cells. Here, we describe a method termed directed evolution of reprogramming factors by cell selection and sequencing (DERBY-seq) to identify artificially enhanced and evolved reprogramming transcription factors. DERBY-seq entails pooled screens with libraries of positionally randomised genes, cell selection based on phenotypic readouts, and genotyping by amplicon sequencing for candidate identification. We benchmark this approach using pluripotency reprogramming with libraries based on the reprogramming factor SOX2 and the reprogramming incompetent endodermal factor SOX17. We identified several SOX2 variants outperforming the wild-type protein in three- and four-factor cocktails. The most effective variants were discovered from the SOX17 library, demonstrating that this factor can be converted into a highly potent inducer of pluripotency with a range of diverse modifications. We propose DERBY-seq as a broad-based approach to discover reprogramming factors for any donor/target cell combination applicable to direct lineage reprogramming in vitro and in vivo., Graphical Abstract, Highlights • Pooled screens with site-randomized reprogramming factor libraries • The phenotype of cells is used to isolate factors with enhanced functions • Native factors are not optimized for non-native reprogramming conditions • Artificially evolved factors accelerate reprogramming, Transcription factor-driven cell-fate conversions are powerful methods to turn one cell type into another but are typically slow and inefficient. In this article Jauch and Veerapandian et al. showed that naturally occurring factors are not optimally adapted for this feat but can be profoundly improved by artificially evolving the way they read and translate regulatory information stored in the genome.

During mouse embryo development, both muscle progenitor cells (MPCs) and brown adipocytes (BAs) are known to derive from the same Pax7+/Myf5+ progenitor cells. However, the underlying mechanisms for the cell fate control remain unclear. In Pax7-null MPCs from young mice, several BA-specific genes, including Prdm16 and Ucp1 and many other adipocyte-related genes, were upregulated with a concomitant reduction of Myod and Myf5, two muscle lineage-determining genes. This suggests a cell fate switch from MPC to BA. Consistently, freshly isolated Pax7-null but not wild-type MPCs formed lipid-droplet-containing UCP1+ BA in culture. Mechanistically, MyoD and Myf5, both known transcription targets of Pax7 in MPC, potently repress Prdm16, a BA-specific lineage-determining gene, via the E2F4/p107/p130 transcription repressor complex. Importantly, inducible Pax7 ablation in developing mouse embryos promoted brown fat development. Thus, the MyoD/Myf5-E2F4/p107/p130 axis functions in both the Pax7+/Myf5+ embryonic progenitor cells and postnatal myoblasts to repress the alternative BA fate.