Cardiac-specific Deletion Of ErbB4 In The Adult Mouse Increases Cardiac Cell Proliferation And Causes Physiological Cardiac Hypertrophy

The epidermal growth factor receptor type 4 (ErbB4) is a receptor tyrosine kinase that is activated by the growth factor neuregulin 1 (NRG1). ErbB4 is essential for cardiac development and promotes cardiomyocyte survival in culture. To investigate the physiological importance of ErbB4 in the adult heart, we generated a mouse model with conditional deletion of ErbB4 in cardiomyocytes using a tamoxifen-inducible Cre recombinase (MHC-MerCreMer). Adult MHC-MerCreMer/ErbB4 fl/fl (ErbB4 conditional knockout, cKO, n=6) and MHC-MerCreMer/ErbB4 WT/WT (control, n=8) animals were injected with tamoxifen (20 mg/kg/day ip for 10 days). Ten days after tamoxifen treatment, cardiac specific deletion of ErbB4 was confirmed by qPCR. At 3 months after ErbB4 deletion, echocardiography showed no differences in cardiac function (fractional shortening), however the heart weight:tibia length ratio was increased by 45% in the cKO animals (5.1±0.2 vs 7.4±0.6 mg/mm, P <0.05 vs controls). Cardiac-specific ErbB4 deletion did not increase expression of genes associated with pathological hypertrophy (BNP, α-MHC:β-MHC ratio), suggesting that the hypertrophy seen in the cKO animals may be physiological rather than pathological. Similarly, cardiac-specific ErbB4 deletion did not cause cardiac fibrosis (Masson’s Trichrome) or alter fibrotic gene expression (Col1A1, Col3A1 and PAI1). Cardiomyocyte cross sectional area was measured using wheat germ agglutinin staining, and showed no differences between controls and cKO. Interestingly, cardiac mRNA levels of both isoforms of the endogenous ErbB4 agonist NRG1 were strongly increased in cKO animals (fold change compared to controls: 5.3±1.7 for NRG1α; 4.3±1.2 for NRG1β, P <0.01) and this correlated with a significant increase in the total number of cells positive for a marker of proliferation (phosphorylated-histone H3; 1.8±0.4 vs 8.2±2.8 cells/section, P <0.05). Conclusion: Cardiac-specific deletion of ErbB4 in the adult increases heart weight without altering cardiac function, cardiomyocyte size or fibrosis. ErbB4 deletion also increases NRG1 expression, which may contribute to the maintenance of cardiac function and the development of cardiac hypertrophy via cell proliferation.