Neuregulin1 regulates cardiomyocyte dynamics, proliferation, and maturation during ventricular chamber morphogenesis

BACKGROUND: Cardiac ventricles are essential for providing the contractile force of the beating heart throughout life. How the primitive endocardium-layered myocardial projections called trabeculae form and mature into the adult ventricles is of great interest for fundamental biology and regenerative medicine. Trabeculation is dependent on the signaling protein Neuregulin1 (NRG1). However, the mechanism of action of NRG1 and its role in ventricular wall maturation are poorly understood. METHODS: The function and downstream mechanisms of NRG1 signaling during ventricular chamber development were explored in mice with conditional cardiac-specific-Nrg1 depletion or overexpression using confocal imaging, transcriptomics, and biochemical approaches. RESULTS: Analysis of cardiac-specific-Nrg1 mutant mice showed that the apicobasal polarity transcriptional program underlying cardiomyocyte-oriented cell division and trabeculae formation depends on endocardial NRG1 to myocardial ERBB2 signaling and pERK activation. Early endothelial loss of NRG1 reduced cardiomyocyte PAR3 expression and apical-domain expression of CRUMBS, caused N-CADHERIN mis-localization, and altered actin cytoskeletal organization. Impaired trabeculation in Nrg1 mutants was associated with a directional shift from perpendicular to parallel/obliquely-oriented cardiomyocyte division. Later endothelial NRG1 depletion resulted in ventricular hypoplasia and lack of an inner compact myocardial wall. Gene profiling indicated that NRG1 is required for trabecular growth and ventricular wall thickening by regulating an epithelial-to-mesenchyme transition (EMT)-like process in cardiomyocytes involving migration, adhesion, cytoskeletal actin turnover, and timely progression through the cell cycle. Cardiac NRG1 overexpression and pERK hyperactivation maintained high EMT-like gene expression and prolonged the trabeculation phase, hindering the formation and maturation of the compact myocardial wall. Likewise, alterations to myocardial trabecular patterning resulting from above- or below-normal NRG1 expression were concomitant with profound disorganization of the sarcomere actin cytoskeleton. The NRG1 loss- and gain-of-function transcriptomes were enriched for an EMT-like gene signature associated with yes-associated protein-1 (YAP1), identifying YAP1 as a potential downstream effector. Biochemical and imaging data showed that pERK activation and nuclear-cytoplasmic distribution of YAP1 during trabeculation are dependent on NRG1. CONCLUSIONS: These data establish NRG1 as a crucial orchestrator of cardiomyocyte proliferation and migration during ventricular development and identify a NRG1-dependent signaling cascade that could be leveraged for future cardiac regenerative therapies. ### Competing Interest Statement The authors have declared no competing interest.