Heart unloading boosts cardiomyocyte regeneration via epicardial NRG1–ERBB4 pathway

For more than a century, the adult human heart has been viewed as a terminally differentiated organ, incapable of meaningful self-repair. Cardiomyocytes, the contractile cells that make up the bulk of cardiac tissue, exit the cell cycle shortly after birth and never meaningfully return. When a heart attack destroys a section of muscle, the organ responds with scarring rather than regeneration, setting patients on a path toward heart failure. Now, a study published in Nature Cardiovascular Research challenges the permanence of that post-mitotic state, showing that simply taking the load off an adult heart — allowing it to rest rather than pump — can coax mature cardiomyocytes back into the cell cycle and even drive regeneration of damaged tissue.

The research, led by a team using a heterotopic heart transplantation model in mice, provides some of the most direct evidence yet that mechanical forces are a central regulator of cardiac regenerative capacity. In the heterotopic model, a donor heart is surgically connected to the recipient’s circulation in parallel with its own heart. The graft becomes vascularized and receives blood, but because it beats in parallel rather than in series with the native heart, it performs no pumping work. The result is a heart that is alive and perfused but mechanically unloaded — a preparation that has existed for decades as a surgical curiosity but has now been repurposed as a powerful tool for probing mechanotransduction in cardiac regeneration.

Using this system, the investigators tracked cell-cycle re-entry in adult cardiomyocytes with two complementary approaches. First, they employed Ki67-based assays, detecting the expression of Ki67, a nuclear protein present in all active phases of the cell cycle but absent from resting cells, to document that cardiomyocytes in unloaded hearts re-entered the cell cycle at rates far exceeding those observed in normally loaded hearts. Second, and more conclusively, they used MADM — mosaic analysis with double markers — a genetic lineage-tracing technique that allows the daughter cells of a single dividing progenitor to be labeled in distinct, distinguishable colors. Because MADM unambiguously distinguishes true cell division from nuclear doubling or DNA synthesis without cytokinesis, a frequent source of false positives in cardiomyocyte proliferation studies, the lineage-tracing data established that unloaded adult hearts generate new cardiomyocytes through genuine cell division, not merely through cell-cycle activity that stalls before completion.

The team then asked whether this load-dependent regenerative potential could be exploited therapeutically. Applying the same heterotopic transplantation approach to hearts that had suffered myocardial infarction, they found that mechanical unloading promoted regeneration specifically within peri-infarct regions — the border zones where viable, damaged myocardium surrounds the dead scar. This finding is clinically evocative: left ventricular assist devices (LVADs), which unload the failing heart in patients awaiting transplantation or as destination therapy, are known to produce reverse remodeling, and reverse remodeling has been associated with increases in cardiomyocyte cell-cycle markers in human explant tissue. The new work provides a mechanistic framework for understanding what those devices might actually be doing at the cellular level.

To identify the molecular signals linking mechanical load to proliferation, the researchers performed single-nucleus RNA sequencing on unloaded hearts, profiling gene expression in individual nuclei to reconstruct the tissue’s cellular ecosystem. The analysis revealed a striking enhancement of communication between the epicardium — the outer epithelial layer of the heart — and cardiomyocytes. Rather than an intrinsic shift within cardiomyocytes themselves, unloading appeared to reprogram the epicardium, which ramped up production of neuregulin 1 (NRG1), a secreted growth factor long known to promote cardiomyocyte proliferation during embryonic development and in neonatal life.

The pathway that carried the signal was equally revealing. NRG1 acts on cardiomyocytes through its receptor ERBB4, a receptor tyrosine kinase whose downstream signaling had previously been implicated in developmental cardiac growth. In the unloaded heart, single-cell analysis and functional experiments traced a signaling route from epicardial NRG1 through cardiomyocyte ERBB4 to the transcription factor STAT3, which became activated and translocated to drive a proliferation program. The causal chain was tested with genetic precision: when the investigators deleted Nrg1 specifically in the epicardium, STAT3 activation in cardiomyocytes was lost, and cardiomyocyte proliferation in the unloaded hearts was abolished. The experiment demonstrates that the epicardium is not a passive covering but an active mechanosensory relay that translates the absence of load into a proliferative instruction.

The final piece of the mechanism addresses a question that has dogged adult cardiomyocyte proliferation research: why do adult cardiomyocytes that enter the cell cycle so often fail to complete division? The answer, according to the new study, lies in metabolism. Using CUT&Tag, a chromatin profiling technique that maps where a transcription factor binds across the genome and what it does to local histone marks, the team showed that STAT3 directly upregulates the gene H6pd, which encodes hexose-6-phosphate dehydrogenase, an enzyme operating within the endoplasmic reticulum that channels glucose-6-phosphate flux through the oxidative branch of the pentose phosphate pathway. Activation of this metabolic arm boosts the production of nucleotide precursors — the raw material for DNA replication — and of reducing equivalents in the form of NADPH, which buffer the oxidative stress that accompanies proliferative metabolism. In other words, mechanical unloading does not merely flip a mitogenic switch; it simultaneously opens a metabolic supply line that allows the cell-cycle program to run to completion.

Taken together, the findings delineate what the authors describe as a mechanotransductive pathway: mechanical load suppresses regenerative signaling, and mechanical unloading restores it, with the signal flowing from the epicardium through NRG1 to ERBB4 and STAT3 in cardiomyocytes, and onward into metabolic reprogramming that sustains cell division. The pathway’s architecture explains a biological paradox — why neonatal mammals can regenerate their hearts while adults cannot — in terms of workload rather than an irreversible loss of potential. At birth, the transition from placental to pulmonary circulation imposes the full hemodynamic burden of systemic pumping on the heart, and cardiomyocytes respond to that load by withdrawing permanently from the cycle. Remove the load, the new data suggest, and the withdrawal can be partially reversed, at least in mice.

The translational implications extend across several domains of cardiovascular medicine. For patients on LVAD support, the study suggests that unloading therapy may be doing more than resting and remodeling the failing heart; it may be actively stimulating endogenous repair through epicardial NRG1 signaling, and the NRG1–ERBB4 axis offers a pharmacological target for amplifying that effect. Exogenous NRG1 has already been tested in clinical trials for heart failure, with mixed results, and the new mechanistic data suggest that timing, delivery, and the concurrent mechanical state of the heart may determine whether neuregulin therapy succeeds. The metabolic arm of the pathway — the STAT3–H6pd link to the pentose phosphate pathway — similarly suggests that proliferative failure in the adult heart may be a bioenergetic problem that could be addressed directly. The study also reframes surgical and device-based strategies: heterotopic approaches, partial unloading, or even temporally staged unloading could, in principle, be designed to open a regenerative window in injured myocardium, allowing peri-infarct tissue to rebuild before the heart resumes full load.

Important caveats remain. The work was performed in mice, and the heterotopic transplant model is an extreme form of unloading that cannot be directly replicated in patients; the degree, duration, and rhythm of unloading that a human heart would tolerate while retaining regenerative benefit are unknown. The magnitude of cardiomyocyte renewal observed, while functionally meaningful in the peri-infarct setting of the mouse, would need to be substantially amplified to rebuild a large human infarct. And the epicardium, thin and largely quiescent in the adult human heart, may respond differently to unloading than the mouse epicardium does. Nevertheless, the study establishes a clear causal pathway from mechanics to metabolism in cardiac regeneration, and it elevates mechanical load from a passive biomechanical variable to a master regulator of cardiomyocyte cell fate. For a field that has spent decades searching for the molecular key to heart regeneration, the message is unexpectedly physical: one of the most powerful signals for renewal may be rest itself.