Mechanical unloading promotes adult cardiomyocyte proliferation through epicardial NRG1-ERBB4 signaling.

In mammals, the postnatal increase in cardiac workload coincides with the loss of cardiomyocyte proliferative capacity, rendering adult cardiomyocytes permanently post-mitotic. Whether reducing load can restore regenerative potential in the adult heart remains unknown. Here we use a heterotopic heart transplantation model, in which the donor heart is vascularized but nonpumping, to show that mechanical unloading induces adult cardiomyocyte proliferation, revealed by Ki67-based and MADM (mosaic analysis with double markers) lineage tracing. Applying heterotopic transplantation to infarcted hearts to achieve mechanical unloading similarly promoted regeneration within peri-infarct regions. Single-nucleus RNA sequencing of unloaded hearts identified enhanced epicardial-cardiomyocyte communication via the NRG1-ERBB4-STAT3 axis. Epicardial Nrg1 deletion blocked STAT3 activation and cardiomyocyte proliferation. CUT&Tag revealed STAT3 directly upregulates H6pd in cardiomyocytes, boosting pentose phosphate pathway activity to supply nucleotides and reducing equivalents for proliferation. These findings delineate a mechanotransductive pathway linking epicardial signals to cardiomyocyte cell-cycle re-entry, providing a framework for leveraging unloading to promote cardiac regeneration.