A critical role of retinoic acid concentration for the induction of a fully human-like atrial phenotype in hiPSC-CM

Retinoic acid (RA) induces an atrial phenotype in human induced pluripotent stem cells (hiPSC), but expression of atrium-selective currents such as the ultrarapid (IKur) and acetylcholine-stimulated K+ current (IK,ACh) is variable and less than in adult human atrium. We suspected methodological issues and systematically investigated the concentration-dependency of RA. RA treatment increased IKur concentration-dependently from 1.1{+/-}0.54 pA/pF (0 RA) to 3.8{+/-}1.1, 5.8{+/-}2.5 and 12.2{+/-}4.3 at 0.01, 0.1 and 1 {micro}M, respectively. Only 1 {micro}M RA induced enough IKur to fully reproduce human atrial AP shape and a robust shortening of action potentials (AP) upon carbachol. We found that sterile filtration caused substantial loss of RA. We conclude that 1 {micro}M RA appears necessary and sufficient to induce a full atrial AP shape in hiPSC-CM in EHT format. RA concentrations are prone to methodological issues and may profoundly impact success of atrial differentiation.