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Summary: Differentiated cardiomyocytes (CMs) must undergo diverse morphological and functional changes during postnatal development. However, the mechanisms underlying initiation and coordination of these changes remain unclear. Here, we delineate an integrated, time-ordered transcriptional network that begins with expression of genes for cell-cell connections and leads to a sequence of structural, cell-cycle, functional, and metabolic transitions in mouse postnatal hearts. Depletion of histone H2B ubiquitin ligase RNF20 disrupts this gene network and impairs CM polarization. Subsequently, assay for transposase-accessible chromatin using sequencing (ATAC-seq) analysis confirmed that RNF20 contributes to chromatin accessibility in this context. As such, RNF20 is likely to facilitate binding of transcription factors at the promoters of genes involved in cell-cell connections and actin organization, which are crucial for CM polarization and functional integration. These results suggest that CM polarization is one of the earliest events during postnatal heart development and provide insights into how RNF20 regulates CM polarity and the postnatal gene program.

Background: Cardiac pressure overload induces cardiac hypertrophy and eventually leads to heart failure. One distinct feature of pathological cardiac hypertrophy is fetal-gene re-expression, but not every cardiomyocyte exhibits fetal gene re-expression in the diseased heart. Adult cardiomyocytes are terminally differentiated cells, so we do not know how the heterogeneity is determined and whether the differential fetal-gene reprogramming indicates a different degree of remodeling among cardiomyocytes. We hypothesized that fetal gene-expressed cardiomyocytes show more pathological features in the pressure-overloaded heart. Results: We induced pressure overload in mice by transverse aortic constriction (TAC) and observed a cardiomyocyte population with expression of β-myosin heavy chain (βMHC, a fetal gene encoded by Myh7) after TAC for 3 days. On transcriptomic and proteomic analyses, βMHC-expressed cardiomyocytes of 3-day TAC hearts were enriched in genes in cardiomyopathy-associated pathways and glycolytic processes. Moreover, results of immunoblotting and enzyme activity assay suggested higher glycolytic activity in βMHC-expressed than non-expressed cardiomyocytes. When we inhibited the glycolytic flux by 2-deoxy-d-glucose, a widely used glycolysis inhibitor, the number of βMHC-expressed cardiomyocytes was reduced, and the level of TEA domain family member 1 (TEAD1), a transcriptional enhancer, was decreased. Also, our spatial transcriptomic results demonstrated that naïve and 3-day TAC hearts had fetal-gene–rich tissue domains that were enriched in pathways in extracellular matrix organization and tissue remodeling. As well, gene levels of glycolytic enzymes were higher in Myh7-positive than Myh7-negative domains. Conclusions: Our data suggest that βMHC-expressed cardiomyocytes progress to pathological remodeling in the early stages of cardiac hypertrophy. In addition, the diverse glycolytic activity among cardiomyocytes might play a role in regulating gene expression via TEAD1 signaling.