The adult mammalian heart possesses little regenerative potential following injury. Fibrosis due to activation of cardiac fibroblasts impedes cardiac regeneration and contributes to loss of contractile function, pathological remodeling and susceptibility to arrhythmias. Cardiac fibroblasts account for a majority of cells in the heart and represent a potential cellular source for restoration of cardiac function following injury through phenotypic reprogramming to a myocardial cell fate. Here we show that four transcription factors, GATA4, Hand2, MEF2C and Tbx5 can cooperatively reprogram adult mouse tail-tip and cardiac fibroblasts into beating cardiac-like myocytes in vitro. Forced expression of these factors in dividing non-cardiomyocytes in mice reprograms these cells into functional cardiac-like myocytes, improves cardiac function and reduces adverse ventricular remodeling following myocardial infarction. Our results suggest a strategy for cardiac repair through reprogramming fibroblasts resident in the heart with cardiogenic transcription factors or other molecules.

Rationale Mammalian cardiomyocytes withdraw from the cell cycle during early post-natal development, which significantly limits the capacity of the adult mammalian heart to regenerate following injury. The regulatory mechanisms which govern cardiomyocyte cell cycle withdrawal and binucleation are poorly understood. Objective Given the potential of microRNAs (miRNAs) to influence large gene networks and modify complex developmental and disease phenotypes, we searched for miRNAs that were regulated during the postnatal switch to terminal differentiation. Methods and Results Microarray analysis revealed subsets of miRNAs that were up- or down-regulated in cardiac ventricles from mice at 1- and 10-days of age (P1 and P10). Interestingly, miR-195 (a member of the miR-15 family) was the most highly up-regulated miRNA during this period, with expression levels almost 6-fold higher in P10 ventricles relative to P1. Precocious over-expression of miR-195 in the embryonic heart was associated with ventricular hypoplasia and ventricular septal defects in βMHC-miR-195 transgenic mice. Using global gene profiling and Argonaute-2 immunoprecipitation approaches, we show that miR-195 regulates the expression of a number of cell cycle genes, including checkpoint kinase 1 (Chek1), which we identify as a highly conserved direct target of miR-195. Finally, we demonstrate that knock-down of the miR-15 family in neonatal mice using locked nucleic acid (LNA)-modified antimiRs is associated with an increased number of mitotic cardiomyocytes and de-repression of Chek1. Conclusions These findings suggest that up-regulation of the miR-15 family during the neonatal period may be an important regulatory mechanism governing cardiomyocyte cell cycle withdrawal and binucleation.

Abstract-Apoptosis has been causally linked to the pathogenesis of myocardial infarction and heart failure in rodent models. This death process is mediated by two central pathways, an extrinsic pathway involving cell surface receptors and an intrinsic pathway using mitochondria and the endoplasmic reticulum. Each of these pathways has been implicated in myocardial pathology. In this review, we summarize recent advances in the understanding of the intrinsic pathway and how it relates to cardiac myocyte death and heart disease. Key Words: apoptosis Ⅲ necrosis Ⅲ cell death Ⅲ mitochondria Ⅲ Bcl-2 Ⅲ caspase Ⅲ death-inducing signaling complex Ⅲ apoptosome Ⅲ ischemia Ⅲ heart failure O ver the past decade, interest in cell death has intensified among scientists in multiple areas of biology and medicine. This fascination has been driven by the discovery that apoptosis is mediated by an ancient program that is hard-wired into all metazoan cells. Renewed attention in the cardiovascular field has been fueled by the notion that cell death is often an active process that, in principle, can be inhibited in various disease states.