The translational landscape of direct cardiac reprogramming reveals a role of Ybx1 in repressing cardiac fate acquisition [scRNA-seq]
Direct reprogramming of fibroblasts into induced cardiomyocytes (iCMs) holds great promise for heart regeneration. Although great 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 using integrative ribosome and transcriptomic profiling, and showed extensive translatome re-patterning during fibroblast conversion into iCM. Loss of function screening for translational regulators uncovered Ybx1 as a critical barrier to iCM induction. Knockdown of Ybx1 dramatically improved the efficiency and quality of iCM reprogramming. Mechanistically, Ybx1 directly bound the transcripts of Srf and Baf60c, both of which mediated, at least partially, the repressive effect of Ybx1 on iCM generation.and Depletion of Ybx1 de-repressed the translation of its targets including SRF and Baf60c. Interestingly, upon removing Ybx1, Tbx5 alone could reprogram fibroblasts into iCMs that exhibit classic iCM molecular features and reprogramming trajectories revealed by our single cell dual-omics. In sum, we presented here a global view of translatome dynamics of cardiac reprogramming and identified a novel translational barrier, Ybx1, to iCM generation. Removing this barrier allowed the single factor mediated iCM reprogramming. Overall design: Single-cell RNA-seq profiles of T+shNT and T+shYbx1 infected  fibroblast were generated at day 12 post infection by 10x Genomics Chromium System.