Combinatorial activin receptor-like kinase/Smad and basic fibroblast growth factor signals stimulate the differentiation of human embryonic stem cells into the cardiac lineage.

The transforming growth factor beta/bone morphogenetic protein-activated Smad signaling pathway plays a complicated role in the maintenance of human embryonic stem cell (hESC) pluripotency and in the cell fate decision of hESCs. Here, we report that sustained inhibition of the transforming growth factor beta type I receptor (also termed activin receptor-like kinase or ALK) using a chemical inhibitor selective for ALK4/5/7 (ALKi) leads to the cardiac differentiation of hESCs under feeder-free and serum-free conditions. Treatment with ALKi reduced Smad2/3 phosphorylation and increased Smad1/5/8 phosphorylation in hESCs, suggesting a requirement for active Smad1/5/8 signaling for cardiac induction in these cells when ALK/Smad2/3 is inhibited. Importantly, active basic fibroblast growth factor (bFGF) signaling was also required for ALKi-mediated cardiac differentiation of monolayer-cultured hESCs. The FGF receptor inhibitor SU5402 blocked ALKi-mediated cardiac induction in hESCs, whereas bone morphogenetic protein-4 enhanced the ALKi-induced increase in phospho-Smad1/5/8 levels but failed to induce the cardiac differentiation of hESCs and instead promoted trophoblastic differentiation. We also confirmed that ALKi potentially enhanced the cardiac differentiation of human embryoid bodies, as determined by expression of cardiac-specific markers, increased beating areas, and action potential recorded from beating areas. These results demonstrate that an ALKi could be used as a potential cardiac-inducing agent and that the development of culture conditions that provide an appropriate balance between ALK/Smad and bFGF signaling is necessary to direct the fate of hESCs into the cardiac lineage.