Background Adult mammalian hearts have a limited ability to generate new cardiomyocytes. Proliferation of existing adult cardiomyocytes (ACM) is a potential source of new cardiomyocytes. Understanding the fundamental biology of ACM proliferation could be of great clinical significance for treating myocardial infarction (MI). We aim to understand the process and regulation of ACM proliferation and its role in new cardiomyocyte formation of post-MI mouse hearts. Methods β-actin-GFP transgenic mice and fate-mapping Myh6-MerCreMer-tdTomato/lacZ mice were used to trace the fate of ACMs. In a co-culture system with neonatal rat ventricular myocytes (NRVMs), ACM proliferation was documented with clear evidence of cytokinesis observed with time-lapse imaging. Cardiomyocyte proliferation in the adult mouse post-MI heart was detected by cell cycle markers and EdU incorporation analysis. Echocardiography was used to measure cardiac function and histology was performed to determine infarction size. Results In-vitro, mononucleated and bi/multi-nucleated ACMs were able to proliferate at a similar rate (7.0%) in the co-culture. Dedifferentiation proceeded ACM proliferation, which was followed by redifferentiation. Redifferentiation was essential to endow the daughter cells with cardiomyocyte contractile function. Intercellular propagation of Ca2+ from contracting NRVMs into ACM daughter cells was required to activate the Ca2+ dependent calcineurin-nuclear factor of activated T cells signaling pathway to induce ACM redifferentiation. The properties of NRVM Ca2+ transients influenced the rate of ACM redifferentiation. Hypoxia impaired the function of gap junctions by dephosphorylating its component protein connexin 43, the major mediator of intercellular Ca2+ propagation between cardiomyocytes, thereby impairing ACM redifferentiation. In-vivo, ACM proliferation was found primarily in the MI border zone. An ischemia resistant connexin 43 mutant enhanced the redifferentiation of ACM-derived new cardiomyocytes after MI and improved cardiac function. Conclusions Mature ACMs can reenter the cell cycle and form new cardiomyocytes through a three-step process, dedifferentiation, proliferation and redifferentiation. Intercellular Ca2+ signal from neighboring functioning cardiomyocytes through gap junctions induces the redifferentiation process. This novel mechanism contributes to new cardiomyocyte formation in post-MI hearts in mammals.

BackgroundHypoxia/reoxygenation(H/R)-induced apoptosis of cardiomyocytes plays an important role in myocardial injury. Lycopene is a potent antioxidant carotenoid that has been shown to have protective properties on cardiovascular system. The aim of the present study is to investigate the potential for lycopene to protect the cardiomyocytes exposed to H/R. Moreover, the effect on mitochondrial function upon lycopene exposure was assessed.Methods and FindingsPrimary cardiomyocytes were isolated from neonatal mouse and established an in vitro model of H/R which resembles ischemia/reperfusion in vivo. The pretreatment of cardiomyocytes with 5 µM lycopene significantly reduced the extent of apoptosis detected by TUNEL assays. To further study the mechanism underlying the benefits of lycopene, interactions between lycopene and the process of mitochondria-mediated apoptosis were examined. Lycopene pretreatment of cardiomyocytes suppressed the activation of the mitochondrial permeability transition pore (mPTP) by reducing the intracellular reactive oxygen species (ROS) levels and inhibiting the increase of malondialdehyde (MDA) levels caused by H/R. Moreover, the loss of mitochondrial membrane potential, a decline in cellular ATP levels, a reduction in the amount of cytochrome c translocated to the cytoplasm and caspase-3 activation were observed in lycopene-treated cultures.ConclusionThe present results suggested that lycopene possesses great pharmacological potential in protecting against H/R-induced apoptosis. Importantly, the protective effects of lycopene may be attributed to its roles in improving mitochondrial function in H/R-treated cardiomyocytes.

Mitochondrial (mt) dysfunction and oxidative stress are involved in the pathogenesis of ischemia/reperfusion (I/R)-injury. Lycopene, a lipophilic antioxidant found mainly in tomatoes and in other vegetables and fruits, can protect mtDNA against oxidative damage. However, the role of mtDNA in myocardial I/R-injury is unclear. In the present study, we aimed to determine if and how lycopene protects cardiomyocytes from I/R-injury. In both in vitro and in vivo studies, I/R-injury increased mt 8-hydroxyguanine (8-OHdG) content, decreased mtDNA content and mtDNA transcription levels, and caused mitochondrial dysfunction in cardiomyocytes. These effects of I/R injury on cardiomycoytes were blocked by pre-treatment with lycopene. MtDNA depletion alone was sufficient to induce cardiomyocyte death. I/R-injury decreased the protein level of a key activator of mt transcription, mitochondrial transcription factor A (Tfam), which was blocked by lycopene. The protective effect of lycopene on mtDNA was associated with a reduction in mitochondrial ROS production and stabilization of Tfam. In conclusion, lycopene protects cardiomyocytes from the oxidative damage of mtDNA induced by I/R-injury.

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After myocardial infarction (MI), the heart is difficult to repair because of great loss of cardiomyoctyes and lack of cardiac regeneration. Novel drug candidates that aim at reducing pathological remodeling and stimulating cardiac regeneration are highly desirable. In the present study, we identified if and how a novel porcupine inhibitor CGX1321 influenced MI and cardiac regeneration. Permanent ligation of left anterior descending (LAD) coronary artery was performed in mice to induce MI injury. Cardiac function was measured by echocardiography, infarct size was examined by TTC staining. Fibrosis was evaluated with Masson's trichrome staining and vimentin staining. As a result, CGX1321 administration blocked the secretion of Wnt proteins, and inhibited both canonical and non-canonical Wnt signaling pathways. CGX1321 improved cardiac function, reduced myocardial infarct size, and fibrosis of post-MI hearts. CGX1321 significantly increased newly formed cardiomyocytes in infarct border zone of post-MI hearts, evidenced by the increased EdU cardiomyocytes. Meanwhile, CGX1321 increased Ki67 and phosphohistone H3 (PH3) cardiomyocytes in culture, indicating enhanced cardiomyocyte proliferation. The mRNA microarray showed that CGX1321 up-regulated cell cycle regulating genes such as and CGX1321 did not alter YAP protein phosphorylation and nuclear translocation in cardiomyocytes. In conclusion, porcupine inhibitor CGX1321 reduces MI injury by limiting fibrosis and promoting regeneration. It promotes cardiomyocyte proliferation by stimulating cell cycle regulating genes with a Hippo/YAP-independent pathway.