Background Adult mammalian cardiomyocytes (CM) have the potential to proliferate, but this is not sufficient to generate adequate CMs after myocardial infarction (MI). The transcription factor Tbx20 is required for CM proliferation during development and adult CM homeostasis. The ability of Tbx20 overexpression (Tbx20OE) to promote adult CM proliferation and improve cardiac function after MI was examined. Methods and Results Tbx20OE was induced specifically in adult mouse differentiated CMs. Increased CM proliferation and fetal-like characteristics were found in Tbx20OE hearts, compared to controls, without causing pathology 4 weeks after Tbx20OE at baseline. Moreover, Tbx20OE in adult CM after MI significantly improved survival, cardiac function, and infarct size 4 weeks post-MI. Improved cardiac repair, as indicated by increased CM proliferation and capillary density, was observed in the MI border zone of Tbx20OE hearts compared to controls. Expression of proliferation activator (cyclin D1, E1, and IGF1) and fetal contractile protein (ssTNI, βMHC) mRNA was increased, while negative cell-cycle regulators (p21, Meis1) were decreased, in Tbx20OE hearts, compared to controls, under both baseline and MI conditions. Tbx20OE in adult hearts activates multiple pro-proliferation pathways including Akt, YAP and BMP. Interestingly, p21, Meis1, and a novel cell-cycle inhibitory gene Btg2, are directly bound and repressed by Tbx20 with induction of proliferation in neonatal CM. Conclusions Tbx20OE specifically in adult CM activates multiple cardiac proliferative pathways, directly represses cell-cycle inhibitory genes p21, Meis1 and Btg2, promotes adult CM proliferation, and preserves cardiac performance post-MI.

Cardiac fibrosis is characterized by excessive extracellular matrix deposition that contributes to compromised cardiac function and potentially heart failure. Cardiac pressure overload resulting from trans-aortic constriction in mice leads to cardiac fibrosis and increased Wnt/β-catenin signaling in cardiac fibroblasts. Here, we conditionally induce β-catenin loss of function in resident cardiac fibroblasts using Tcf21 MerCreMer or in activated cardiac fibroblasts using periostin (Postn)MerCreMer. We show that β-catenin loss of function in cardiac fibroblasts after trans-aortic constriction significantly preserves cardiac function, and reduces interstitial fibrosis but does not alter the numbers of activated or differentiated cardiac fibroblasts in vivo. However, β-catenin is specifically required in resident cardiac fibroblasts for fibrotic excessive extracellular matrix gene expression and binds Col3a1 and Postn gene sequences in cultured cardiac fibroblasts after induction of Wnt signaling. Moreover, cardiomyocyte hypertrophy is blunted with cardiac fibroblast-specific loss of β-catenin after trans-aortic constriction in vivo. Thus, Wnt/β-catenin signaling in resident cardiac fibroblasts is required for excessive extracellular matrix gene expression and collagen deposition after trans-aortic constriction.

Background-Neuronal nitric oxide synthase (nNOS) is expressed in cardiomyocytes and plays a role in regulating cardiac function and Ca 2ϩ homeostasis. However, the role of nNOS in cardiac electrophysiology after myocardial infarction (MI) is unclear. We hypothesized that nNOS deficiency increases ventricular arrhythmia and mortality after MI.