Translational landscape of direct cardiac reprogramming reveals a role of Ybx1 in repressing cardiac fate acquisition.

Direct reprogramming of fibroblasts into induced cardiomyocytes holds great promise for heart regeneration. Although considerable progress has been made in understanding the transcriptional and epigenetic mechanisms of iCM reprogramming, its translational regulation remains largely unexplored. Here, we characterized the translational landscape of iCM reprogramming through integrative ribosome and transcriptomic profiling, and found extensive translatome repatterning during this process. Loss of function screening for translational regulators uncovered Ybx1 as a critical barrier to iCM induction. In a mouse model of myocardial infarction, removing Ybx1 enhanced in vivo reprogramming, resulting in improved heart function and reduced scar size. Mechanistically, Ybx1 depletion de-repressed the translation of its direct targets SRF and Baf60c, both of which mediated the effect of Ybx1 depletion on iCM generation. Furthermore, removal of Ybx1 allowed single factor Tbx5-mediated iCM conversion. In summary, this study revealed a new layer of regulatory mechanism that controls cardiac reprogramming at the translational level.

Cross-lineage potential of Ascl1 uncovered by comparing diverse reprogramming regulatomes.

Direct reprogramming has revolutionized the fields of stem cell biology and regenerative medicine. However, the common mechanisms governing how reprogramming cells undergo transcriptome and epigenome remodeling (i.e., regulatome remodeling) have not been investigated. Here, by characterizing early changes in the regulatome of three different types of direct reprogramming, we identify lineage-specific features as well as common regulatory transcription factors. Of particular interest, we discover that the neuronal factor Ascl1 possesses cross-lineage potential; together with Mef2c, it drives efficient cardiac reprogramming toward a mature and induced cardiomyocyte phenotype. Through ChIP-seq and RNA-seq, we find that MEF2C drives the shift in ASCL1 binding away from neuronal genes toward cardiac genes, guiding their co-operative epigenetic and transcription activities. Together, these findings demonstrate the existence of common regulators of different direct reprogramming and argue against the premise that transcription factors possess only lineage-specific capabilities for altering cell fate - the basic premise used to develop direct reprogramming approaches.