Paracrine promotion of cardiomyogenesis in embryoid bodies by LIF modulated endoderm

In the vertebrate embryo the heart is the first organ to form. Embryonic an d extra-embryonic tissues are supposed to contribute to cardiac lineage com mitment before and during gastrulation in a paracrine fashion. Evidence has accumulated that factors secreted by the anterior lateral endoderm and ext ra-embryonic endoderm contribute to cardiomyogenesis. Here we exploit in vi tro differentiation of embryonic stem cells in embryoid bodies to study dif ferentiation of the extraembryonic endodermal lineage, gastrulation-like pr ocesses, and the influence of endoderm on cardiomyogenesis. We demonstrate that in embryoid bodies primitive endoderm differentiates to visceral and p arietal endoderm and that parietal endoderm influences onset of cardiomyoge nesis in a concentration-dependent manner. Both increased concentrations of leukemia inhibitory factor and its absence in lif -/- embryoid bodies hamp ered parietal endoderm formation. Reduced differentiation of parietal endod erm correlated with an attenuation of cardiomyogenesis even in the presence of LIE These and previous results suggest that leukemia inhibitory factor is directly and indirectly, via endoderm formation, involved in the regulat ion of cardiomyogenesis. Increased proliferation of parietal endoderm in li fr -/- embryoid bodies and addition of conditioned lif -/- cell culture sup ernatant promoted cardiomyogenesis, demonstrating for the first time that p arietal endoderm also contributes to cardiomyogenesis in embryoid bodies in a paracrine and leukemia inhibitory factor and its receptor independent pa thway. New factors signaling independently of the leukemia inhibitory-facto r receptor pathway may sustain cardiomyocyte cell proliferation and thus be a future target for gene therapy of cardiomyopathies and cell therapy of t he myocardium.