Abstract-Embryonic stem cells represent an attractive source of cardiomyocytes for cell-replacement therapies. However, before embryonic stem cells can be successfully used for the treatment of cardiac diseases, the precise molecular mechanisms that underlie their cardiogenic differentiation must be identified. A network of intrinsic and extrinsic factors regulates embryonic stem cell self-renewal and differentiation into a variety of different cell lineages. Here, we show that Notch signaling takes place in some but not all embryonic stem cells and that the Notch pathway is shut down during the course of differentiation concomitantly with downregulation of Notch receptor and ligand expression. Moreover, gain-and loss-of-function experiments for Notch signaling components show that this pathway is a crucial regulator of cardiomyocyte differentiation within ES cells. Differentiation of ES cells into cardiomyocytes is favored by inactivation of the Notch1 receptor, whereas endogenous Notch signaling promotes differentiation of ES cells into the neuronal lineage. We conclude that Notch signaling influences the cell fate decision between mesodermal and the neuroectodermal cell fates during embryonic stem cell differentiation. These findings should help to optimize the production of specific cell types via modulation of the Notch pathways and, in particular, to improve the production of embryonic stem cell-derived cardiomyocytes. (Circ Res. 2006;98:1471-1478.)Key Words: Notch Ⅲ embryonic stem cells Ⅲ cardiomyogenesis Ⅲ differentiation Ⅲ gene targeting H eart failure has become the leading cause of death in developed countries. Hundreds of thousands of new cases are diagnosed each year, and despite a large battery of pharmacological agents, heart transplant remains the ultimate therapy for patients with end-stage heart failure. However, the request for organs far exceeds the number of potential donors. As an alternative approach, the regeneration of the myocardium via controlled differentiation of cardiomyocyte progenitors is receiving much attention. Embryonic stem (ES) cells demonstrate several characteristics that suggest that they might serve as a source of cells for the therapeutic regeneration of the heart. First, ES cells can be readily isolated from the inner cell mass of the blastocyst 1,2 and subsequently maintained indefinitely in vitro. [3][4][5] Second, ES cells are totipotent and can be induced to differentiate into a variety of cell types including cardiac myocytes. 6 Third, the recent generation of human ES cell lines has brought further support to the concept of regenerative medicine based on the controlled differentiation of ES cells to replace lost cells in damaged organs. 7 Nevertheless, because of their totipotency, ES cells could paradoxically represent a possible risk of producing teratomas following transfer in vivo. Therefore, knowledge of the precise control of the necessary differentiation processes will be required before ES cells can be used in therapy.The most commonly used method to indu...

Abbreviations used: BNP, brain natriuretic peptide; CPC, cardiac precursor cell; Hes, Hairy/ enhancer of split; MZB, marginal zone B; Sca, stem cell antigen.A. Croquelois, A.A. Domenighetti, and M. Nemir contributed equally to this paper.

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Beta-catenin is a component of the intercalated disc in cardiomyocytes, but can also be involved in signalling and activation of gene transcription. We wanted to determine how long-term changes in beta-catenin expression levels would affect mature cardiomyocytes.Conditional transgenic mice that either lacked beta-catenin or that expressed a non-degradable form of beta-catenin in the adult ventricle were created.While mice lacking beta-catenin in the ventricle do not have an overt phenotype, mice expressing a non-degradable form develop dilated cardiomyopathy and do not survive beyond five months. A detailed analysis could reveal that this phenotype is correlated with a distinct localisation of beta-catenin in adult cardiomyocytes, which cannot be detected in the nucleus, no matter how much protein is present.Our report is the first study that addresses long-term effects of either the absence of beta-catenin or its stabilisation on ventricular cardiomyocytes and it suggests that beta-catenin's role in the nucleus may be of little significance in the healthy adult heart. 3